A complement-fixation (C.F.) test for the human reovirus-like agent of infantile gastroenteritis has been developed using the serologically related Nebraska calf diarrhoea virus (N.C.D.V.) as antigen. Most infants and children who shed the agent in stools and/or who demonstrated serological (C.F.) evidence of infection with a reovirus-like-particle-positive human stool-filtrate C.F. antigen also demonstrated serological evidence of infection when a concentrated N.C.D.V. preparation was employed AS C.F. antigen. The N.C.D.V., which was previously shown to be related to the human reovirus-like agent, was found to be related antigenically to the epizootic diarrhoea of infant mice (E.D.I.M.) virus also. Studies on the prevalence of C.F. antibody in sera from infants and young children revealed a pattern of rapid acquisition of antibody to both the human reovirus-like agent and the N.C.D.V. as over 80 percent of these individuals possessed antibody to each agent by 36 months of age. A strong positive association was found in the results obtained with the two antigens. The ready availability of cell-culture grown N.C.D.V., and its ability to serve as a "substitute" C.F. antigen for the human reovirus-like agent, should enable the serodiagnosis of many cases of disease due to the human agent and facilitate seroepidemiological studies of such infections. In addition, the observation that a large proportion of individuals infected with the human reovirus-like agent develop serological evidence of infection not only to the human agent but to the calf agent as well may have important implications in the immunoprophylaxis of disease caused by the human reovirus-like agent.