The discovery that corticosteroids can alter leucocyte distribution implies that suppression of PFC in the spleen is not necessarily indicative of suppression of the antibody response as a whole. Therefore we have examined IgM and IgG serum antibodies and PFC in the thymus, spleen, femoral marrow, popliteal, thoracic and mesenteric lymph nodes of mice given a single injection of hydrocortisone acetate at various times relative to primary immunization with SRBC. Contrary to certain reports, there was close correlation between suppression of serum antibody and of splenic PFC, from which it was predicted and verified that few PFC could be detected elsewhere. In our hands, therefore, splenic PFC measurements did give an accurate indication of suppression overall. The characteristics of suppression suggest some of the underlying causes, and these are discussed.