Human lymphocytes stimulated for 16 h and then cultured without stimulant showed maximal activity on days 2-3 following a stimulus of phytohaemagglutinin (PHA) and on days 3-4 following a stimulus of staphylococcal filtrate (SF). At low dosage of stimulant the response of the cells was less marked but persisted for a longer period than at high dosage. The pattern of response is discussed in relation to the mechanism of activation.

After the effect of the initial stimulus had died away cell populations which had been stimulated with SF or PHA could be restimulated with either stimulant. Their response, when stimulated this second time, was quicker than that of incubated cells from the same donor which had not been previously stimulated. Prestimulated cells were also tested in two immunospecific reactions : the reaction to tuberculin-purified protein derivative (PPD), and the mixed lymphocyte reaction. Cells which had been previously exposed to SF responded more quickly to PPD than cells not previously stimulated. Cell populations which had been previously stimulated also reacted more quickly in a mixed lymphocyte reaction. It is concluded that lymphocyte populations which have been recently stimulated not only retain their capacity to react to immunospecific mitotic stimuli but also react more quickly.

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