Two inbred strains of Syrian hamster have been shown to display differences in splenic NK cell activity both endogenously and after infection with Pichinde virus. On further investigation of these differences, it was found that the spleens and thymuses of the high NK strain, MHA, displayed greater cellularity than those of the low NK strain, LSH. Additionally, thymocytes from MHA hamsters were found to proliferate to a greater extent than those of LSH hamsters in response to Con A-induced conditioned medium plus lectin. Splenocytes from MHA hamsters showed high levels of lymphokine-activated killer cell (LAK) activity after culture in conditioned medium, in contrast to LSH splenocytes cultured under the same conditions. By limiting dilution analysis, spleens from MHA hamsters were found to contain a greater frequency of LAK precursors than those from LSH hamsters. The data suggest that the relative concentrations of lymphopoietic cells is genetically determined.