The serum concentration of all subclasses of IgG (gamma(2)A, gamma(2)B, and gamma(1)) as well as IgA and IgM were reduced in normal and low pathogen mice receiving hydrocortisone acetate. Turnover studies using (131)I-labeled gamma(2)A subclass of IgG demonstrated that high dose corticosteroids cause a significantly shortened survival (increased catabolic rate) which contributes to the observed hypogammaglobulinemia. This increase in fractional catabolism is not due to excess loss in the urine or stool but reflects an increase in endogenous catabolism.