Isolates of Cryptosporidium parvum obtained from infected humans, calves and lambs were typed using arbitrary primed polymerase chain reaction (AP-PCR) and isoenzyme electrophoresis. All animal isolates tested (n = 17) showed similar profiles in AP-PCR and isoenzyme typing. In AP-PCR assays, 9 out of 15 human isolates showed a distinct "human" profile while the remaining 6 isolates showed the "animal" profile. In isoenzyme typing, 5 human isolates which had shown "human" profiles in AP-PCR demonstrated a unique isoenzyme banding pattern, while 2 isolates which had shown "animal" profiles in AP-PCR gave the "animal" banding pattern. In a murine model of infection, all four animal isolates tested were highly infective but only one of four human isolates identified as "human" type in the AP-PCR and isoenzyme typing systems was infective. The good correlation between the data from the different typing systems supports the hypothesis that there are genetically distinct human and animal populations of C. parvum.