An antigenic mutant of the wild influenza strain A/Texas/1/77 (H3N2) was obtained in vitro by growing virus of the allantosis-on-shell system in the presence of a mouse monoclonal antibody preparation to the Texas haemagglutinin (HA) antigen. A total of 204 human sera, from a population of randomly collected sera during the fall of 1979, were examined by the haemagglutination-inhibition (HI) tests against the Texas wild strain and the antigenic mutant. It was shown that sera from young children (0-5 years) reacted poorly, or not at all, with this mutant strain, whereas sera from adults (15 years +) reacted equally well with both viruses. It can be assumed that the antigenic site recognised by this particular monoclonal antibody preparation (185/1) may be important for the human immune response to the HA antigen of A/Texas/1/77 virus.