Peripheral lymphocyte subsets of 14 heterosexual recipients of hepatitis B vaccine were measured by indirect immunofluorescence using specific monoclonal antibodies. 12/14 vaccine recipients developed antibody to hepatitis B surface antigen after the second injection of vaccine; however, 2 recipients remained seronegative. Prior to the vaccination, these two vaccine non-responders had been found to have a significantly reduced ratio of peripheral helper-inducer T cells to suppressor/cytotoxic T cells (1.15 +/- 0.02 vs. 1.79 +/- 0.12), and an increase in the proportion of natural killer/killer cells (25.6 +/- 1.5% vs. 13.8 +/- 1.6%) as compared to the 12 responders. Enumeration of peripheral lymphocyte subsets prior to vaccination may be useful in predicting the immune response of hepatitis B vaccine recipients.