A comprehensive comparative analysis of lymphocyte populations, natural killer cell activity, and interleukin-2 (IL-2) and interferon production was performed on cord blood and adult peripheral blood mononuclear cell populations. Analysis of the cell types present demonstrated significantly lower percentages of T3+, T8+, and Leu 11+ cells for the cord blood samples compared to the adult samples. Also, both natural killer cell activity and gamma interferon production were significantly lower in the cord blood group. These findings may correlate with the increased susceptibility of young infants to infection. Differences between the two groups were not evident for the percentages of T4+ cells, B cells, and large granular lymphocytes. IL-2 production by cells stimulated with concanavalin A and a phorbol ester was higher in cord blood. Individually, half of the cord blood samples contained more IL-2 than any of the adult samples. This result could be caused either by greater production of IL-2 by the cord blood samples or by greater utilization of IL-2 by the adult samples.