The technique of flow cytofluorometry has been employed to assess the response of unfractionated and highly purified human lymphocyte subpopulations to phytohemagglutinin (PHA) and pokeweed mitogen. Normal values for cytofluorometric responses were established and compared to the uptake of tritiated thymidine in simultaneous experiments. Cytofluorometric analysis offered the advantages of increased sensitivity and direct measurement of DNA content per cell, and provided percentages and absolute numbers of responding cells. B-cell responses to pokeweed mitogen were absent, but brisk T-cell responses were noted. Between 4% and 8% of highly purified human B cells were found to respond to PHA by increasing their DNA content; modest but significant uptake of tritiated thymidine by B cells following PHA stimulation was also observed.