Hybridoma antibodies (HAbs) against oocyst antigens of a human isolate of Cryptosporidium parvum were developed by fusion of SP2/0 mouse myeloma cells and spleen cells from BALB/c mice immunized with oocyst homogenates. In an indirect immunofluorescence antibody test (IFAT), using as antigen a mixture of air-dried sporozoites and oocysts, HAbs labelled either the oocyst wall or areas of the sporozoite, including the whole organism, the entire surface, a polar region or the interior. Most of the HAbs were specific for the sporozoite surface, and few of them recognized the oocyst wall. In Western blot analysis of oocyst antigens, sporozoite surface-reactive monoclonal antibodies (MoAbs) recognized one or more of seven polypeptide bands with molecular weights in the range 47- greater than 200 kD, and all reacted with the 47 kD band. Each of four heterologous parasite isolates had a unique recognition pattern with a panel of MoAbs in IFAT, suggesting antigenic differences may exist between strains of C. parvum. The ability to differentiate between parasite isolates by immunological methods might be of value in epidemiological studies of cryptosporidiosis.