Polymerase chain reaction (PCR) and newer serologic assays for hepatitis C virus (HCV) were used to investigate 19 HCV cross-challenge episodes in chimpanzees. In these cross-challenges, 59% showed seroconversion after challenge, 33% showed reappearance of HCV-associated hepatocellular ultrastructural changes, 5 animals not PCR-positive at the time of challenge showed return ofPCR positivity, and 26% developed hepatitis after rechallenge. A total of 74% showed at least one of these signs of reinfection. The frequency of development of serologic and ultrastructural responses was, however, reduced in secondary compared with primary infections (P < .01). In 10 animals, the cross-challenge was done with heterologous strains, and in 9 with the originally infecting virus. There was no significant difference in the responses to homologous and heterologous challenges. The data suggest relatively weak immunity in HCV infections.