Traditionally, immunotoxicological studies in the rat have been performed by measuring the effect of chemical substances on spleen lymphocytes in vivo and in vitro. However, rat blood lymphocytes may be more relevant than spleen cells for comparison with human blood lymphocytes. Further, lymphocytes in blood may be a more sensitive indicator of immunotoxic effects than spleen lymphocytes. Finally, in longitudinal studies peripheral blood specimens can be collected repeatedly from the same animals, thereby reducing the number of animals sacrificed and, possibly, experimental variation. We compared blood and spleen lymphocyte parameters in rats treated with a single dose of the immunosuppressant cyclophosphamide (CY), monitoring effects on blood and spleen lymphocytes by immunophenotyping. We also performed repeated bleedings to demonstrate the feasibility of following the time course of induced changes in the same animals. Immunophenotyping as well as total mononuclear cell counts consistently showed as large or lager effects of CY in blood lymphocytes than in spleen cells. Further, the measured effects in blood lymphocytes became statistically significant at an earlier time point, compared to spleen cells. Repeated bleedings of the same animals illustrated that blood specimens drawn from a peripheral vein give sufficient numbers of cells to perform immunotoxicological tests.