Sporozoites and merozoites are stages in the life cycle of Cryptosporidium parvum that can cyclically infect intestinal cells, causing persistent infection and severe diarrhea in immunodeficient patients. Infection by sporozoites can be neutralized by surface-reactive mAb. We show that merozoite infectivity can also be neutralized by surface-reactive mAb. To do this, viable C. parvum merozoites were isolated by differential and isopycnic. centrifugation, and distinguished from sporozoites by transmission electron microscopy. Differential reactivity with a panel of seven mAb was used to determine the amount of sporozoite contamination in isolated merozoite preparations. The isolated merozoites were distinguished from sporozoites (p less than 0.0001) by four sporozoite-specific mAb (16.332, 16.502, 17.25, and 18.357) in an indirect immunofluorescence assay. Three mAb (16.29, 17.41, and 18.44) consistently reacted with both merozoites and sporozoites. Isolated merozoites were infectious for neonatal mice when administered by intraintestinal injection. Infectivity for mice was significantly neutralized (p less than 0.05) when 1 to 2 x 10(5) merozoites were incubated with sporozoite-neutralizing mAb 17.41 or 18.44, before inoculation. Merozoites incubated with an isotype control mAb remained infectious for neonatal mice. We conclude that C. parvum merozoites share neutralization-sensitive epitopes with sporozoites.