Three strains of Hong Kong influenza virus isolated from humans were compared with a strain isolated from a calf for their ability to cause disease in calves. One of the human strains, A/Aichi/2/68, was detected for 5 days in a calf, but all 3 failed to cause signs of disease. Strain A/calf/Duschanbe/55/71 could be detected for 7 days and caused an influenza-like illness in calves. [The significance of animals as reservoirs of human influenza viruses was of concern for this study.].