We assessed lymphoproliferative responsiveness of lymphocytes from the spleen and lymph nodes of inbred neonatal SWR/J H-2q mice at various times postinoculation (PI) using Cryptosporidium parvum oocysts. The lymphocytes were cultured in vitro with a water-soluble (SO) and a water-insoluble (IN) antigen fraction. The IN fraction was prepared by solubilizing particulates, the sediment obtained after centrifuging disrupted oocysts, in urea. Both fractions were characterized using silver stain and enzyme-linked immunoelectrotransfer blot (EITB) with hyperimmune rabbit anti-oocyst serum, monoclonal antibody specific to a 23-kD antigen, and serum from patients with symptoms of cryptosporidiosis. The EITB showed that the antigens in the IN fraction differed both quantitatively and qualitatively from those in the SO fraction. Lymphocytes from lymph nodes of exposed mice cultured with the SO fraction exhibited a significant (P < 0.05) and antigen-specific response compared with those from unexposed mice at days 10, 19, 22, and 28 PI. The response to the IN fraction of lymphocytes from lymph nodes of exposed mice was not as consistent as that to the SO fraction but showed a significant (P < 0.05) and antigen-specific response at days 10 and 19 PI. No significant response occurred when splenic lymphocytes were cultured with SO or IN fractions. These results show that lymphocytes from lymph nodes of mice exposed to Cryptosporidium parvum oocysts proliferate when cultured in vitro with soluble or particulate antigens prepared from oocysts.