Human B and T lymphocytes differ in the rate of cell proliferation and frequency of sister chromatid exchange (SCE) when cultured separately in short-term cultures. This difference could theoretically be responsible for part of the variation in the SCE-frequency previously observed among healthy subjects since there is individual variation in the proportion of B and T cells in the peripheral blood. We have therefore studied cell proliferation and SCE-frequency in conventional short-term cultures of lymphocytes from 28 healthy subjects with different proportions of B and T cells. The percentage of B or T lymphocytes did not correlate with the SCE-frequency, nor with the rate of cell proliferation in culture. However, a significantly higher SCE-frequency was found in slowly proliferating cultures than in cultures with a high rate of turn over. Thus, the rate of cell proliferation appears to be an important determinant of the SCE-frequency in conventional lymphocyte cultures. Although the data do not exclude attribution of the difference in SCE- frequency between rapidly and slowly growing cultures to differences in subpopulations of lymphocytes, it appears less likely that B and T cells constitute these tentative subpopulations.