The effects of methylnitrosourea on the immune system and hematopoietic system of adult specific pathogen free cats. 
The effects of a single non-carcinogenic dose of 15 mg/kg methylnitrosourea (MNU) on the immune and hematopoietic systems of adult specific-pathogen-free (SPF) cats were determined.  The cell-mediated-immune (CMI) system was markedly suppressed, as evidenced by: (i) Prolonged cutaneous allograft retention time (41-84 days); (ii) Decreased lymphocyte blast transformation response to mitogens (2% of pretreatment response to pokeweed mitogen or concanavalin A) and antigen (12% of untreated control cat response to keyhole limpet hemocyanin); (iii) Reduced number of absolute erythrocyte-rosetting T-cells in the peripheral blood.  This immunosuppression lasted at least 3 months, the duration of the experiment.  Suppression of the hematopoietic system was also noted as evidenced by: (i) Peripheral lymphopenia lasting 3 months and neutropenia lasting 3 weeks; (ii) Bone marrow hypocellularity lasting 3 weeks; (iii) Hypoplasia of neutrophilic precursors lasting 3 weeks and erythroid precursors lasting 4 days.  It was concluded that a single non-carcinogenic dose of MNU induces a prolonged suppression of the CMI system and a brief suppression of hematopoiesis in adult SPF cats.  The immunosuppression may in part be responsible for the previously observed increased susceptibility to feline leukemia virus infection and disease of adult SPF cats treated with MNU.