Fusion between P3 x 63 Ag8 mouse myeloma cells and spleen cells from BALB/c mice immunized with influenza type A or B or parainfluenza type 1 virus generated reproducibly antiviral antibody-producing somatic cell hybrids (hybridomas). Eleven hybridomas derived from spleen cells of mice immunized with influenza type A virus were directed against the viral hemagglutinin, one reacted with a host component derived from chickens, and one expressed a specificity not further characterized. The hybridoma antibodies tended to be highly specific for the hemagglutinin of the immunizing virus and seemed to express the same repertoire of strain-specific antibody reactivities as splenic precursor B cells, they did not express any of the frequently occurring crossreactive anti-hemagglutinin specificities. Hybridomas producing crossreactive antibodies against hemagglutinin could be obtained if priming and boosting virus were heterologous.