The present study was undertaken to determine if infection of immunocompetent adult C57BL/6N mice with Cryptosporidium parvum would render them more resistant to a challenge infection following immunosuppression with dexamethasone (DEX). Fecal oocyst shedding and parasite colonization of the terminal ilea were greater in immunosuppressed mice than in nonimmunosuppressed mice. Secondary infections with C. parvum resulted in decreased oocyst shedding and reduced parasite colonization compared with primary infections. Flow cytometry revealed fewer splenic B cells but more splenic total T, CD4+ T, CD8+ T cells and macrophages in immunosuppressed mice than in nonimmunosuppressed mice. The CD4+ to CD8+ T cell ratios and blastogenic responses to lipopolysaccharide (LPS), but not to concanavalin A, were decreased in immunosuppressed mice compared with nonimmunosuppressed mice. Blastogenic responses to LPS and percentages of splenic total B cells and macrophages were increased in secondary infections compared to primary infections. Enhanced susceptibility to C. parvum infection in immunosuppressed mice revealed DEX-mediated effects on both cell-mediated and humoral immunity. Our results suggest that increased resistance in immunosuppressed mice to secondary infections with C. parvum may involve increases in B cells and macrophages.