AbstractOffspring of mice treated with cyclophosphamide (Cy; 1, 2.5 or 5 mg/kg) during pregnancy (6-18 days of gestation) and tested for immunocompetence from 5 to 10 weeks of age were found to have defective reticuloendothelial clearance. The main effects were: a) increased elimination half time (T 1/2) of51Cr-labeled SRBC from circulation, b) decreased liver uptake of51Cr and c) impaired ability of the spleen, mostly affecting the female pups, to compensate for decreased liver uptake. The highest dose group suffered the most pronounced effects. This group was also found to have increased IgG immunoglobulin levels at 7 weeks of age. IgG antibody production in response to specific antigenic stimulation and delayed hypersensitivity reactions to oxazolone did not appear to be affected by Cy treatment.