In order to determine the comparability of human and rodent in vitro systems, the direct effects of various therapeutic or environmental chemicals on proliferative responses of lymphocytes of mouse, rat, and human origins were examined and analyzed by a detailed statistical approach. Four compounds of diverse structure and mechanism of action which are known to impair lymphocyte transformation, such as hydroquinone, T-2 toxin, lead nitrate, as well as the widely used immunosuppressive drug cyclosporin A, were chosen as model test substances. T cells were stimulated by phytohaemagglutinin as well as monoclonal antibodies directed at the T cell receptor/CD3 complex, while B cells were activated by the T-independent mitogens, including Staphylococcus aureus cells, Escherichia coli lipopolysaccharide, and Salmonella typhimurium mitogen with specificity for human, mouse, and rat lymphocytes, respectively. In almost all cases the chemicals altered lymphoproliferative responses in a concentration-related manner in all three species. In general, overall similarities in the relative sensitivity of lymphoblastogenesis were obtained when the human dose-response curves were compared to the rodent response curves. Frequent, statistically significant species-dependent discrepancies of the overall response curves between mice and rats were observed. Large, statistically significant differences were observed for inorganic lead, revealing obvious divergences of the effect patterns in all cases, across all species. In this case, rodent species, especially the rat, were very sensitive to immunomodulation by lead, whereas human cells were relatively resistant. It is suggested that direct interspecies comparisons of immunological effects due to chemical treatment in vitro can provide a greater understanding of the relationship between animal and human data, which will improve the confidence of extrapolation from findings in laboratory animals to human health risk.