Frederick Griffith (1877–1941) was a British bacteriologist whose focus was the epidemiology and pathology of bacterial pneumonia. In January 1928 he reported what is now known as Griffith's experiment, the first widely accepted demonstrations of bacterial transformation, whereby a bacterium distinctly changes its form and function.

He showed that Streptococcus pneumoniae, implicated in many cases of lobar pneumonia, could transform from one strain into a different strain. The observation was attributed to an unidentified underlying principle, later known in the Avery laboratory as the "transforming principle" (abbreviated as T. P.) and identified as DNA. America's leading pneumococcal researcher, Oswald T. Avery, speculated that Griffith had failed to apply adequate controls. A cautious and thorough researcher, and a reticent individual, Griffith's tendency was to publish only findings that he believed truly significant, and Griffith's findings were rapidly confirmed by researchers in Avery's laboratory. His discovery was one of the first to show the central role of DNA in heredity.

Frederick Griffith was born in Prescot, Merseyside (formerly in Lancashire) England, in late 1877 (Registered December quarter in Prescot, Lancashire registration district, vol 8b, page 670), and attended Liverpool University. Thereafter, he worked at the Liverpool Royal Infirmary, the Joseph Tie Laboratory, and the Royal Commission on Tuberculosis. In 1910 Fred Griffith was hired by the local government board.

During World War I (1914–18), the local government board's laboratory was assumed by the national government, namely UK government, and became the Ministry of Health's Pathological Laboratory—where Griffith was medical officer. UK government spent money sparingly on the laboratory, which remained very basic, though Griffith and his colleague, William M. Scott, "could do more with a kerosene tin and a primus stove than most men could do with a palace".

Griffith was sent pneumococci samples taken from patients throughout the country, amassed a large number, and would type—in other words classify—each pneumococci sample to search patterns of pneumonia epidemiology, and Griffith experimented on mice for improved understanding of its pathology. Griffith performed the pivotal experiments—actually very many experiments—during the 1920s.

With outbreak of World War II (1939–45), the laboratory was expanded into the Emergency Public Health Laboratory Service.

Pneumococci has two general forms—rough (R) and smooth (S). The S form is more virulent, and bears a capsule, which is a slippery polysaccharide coat—outside the peptidoglycan cell wall common among all classical bacteria—and prevents efficient phagocytosis by the host's innate immune cells. Injected subcutaneously with S form, mice succumbed to pneumonia and death within several days. However, the R form, lacking a capsule—its outer surface being cell wall—is relatively avirulent, and does not cause pneumonia as often.

When Griffith injected heat-killed S into mice, as expected, no disease ensued. When mice were injected with a mixture of heat-killed S and live R, however, pneumonia and death ensued. The live R had transformed into S—and replicated as such—often characterized as Griffith's Experiment. More accurately, point six of Griffith's abstract reports that R tended to transform into S if a large amount of live R, alone, were injected, and that adding much heat-killed S made transformation reliable Griffith also induced some pneumococci to transform back and forth.