At a time when outbreaks and sporadic cases of influenza caused a A/Victoria/3/75-like and A/Texas/1/77-like H3N2 strain of influenza were occurring in the Rocky Mountain region of the USA, about 60% of the students of a high school in Cheyenne, Wyoming, were involved in an outbreak of influenza-like illness. Six influenza A(H1N1) virus isolates were obtained from throat swabs collected from 12 of these students. Virus isolated from a seventh student, however, contained a mixture of H1 and H3 (A/Victoria/3/75-like) hemagglutinins and N1 and N2 neuraminidases, as shown by the ability to clone from the mixture viruses with antigenic components H1N1, H3N1, and H3N2. An antigenic hybrid virus with H3N1 composition was re-isolated from the original throat swab. The results show that one student was shedding a mixture of A/Victoria/3/75(H3N2)-like and A/USSR/90/77(H1N1)-like viruses at the time his throat swab was taken.