The influenza virus A/turkey/Oregon/71 (H7N3) has been adapted to grow in MDCK or chicken embryo cells (CEC) in the absence of trypsin. Changes occurred in the biological properties of the virus variants selected, depending on the cell type used for adaptation. They coincided with enhanced hemagglutinin (HA) activation by intracellular proteolytic cleavage. In the case of MDCK cell selected variants growth, plaque formation, and HA cleavability were restricted to this cell type, whereas the CEC-derived variants displayed altered activities in a broad range of host cells. Unlike the wild-type virus and its MDCK cell-derived variants, CEC variants had acquired pathogenic properties for chickens. By nucleotide sequence analysis of the HA genes of the MDCK cell variants several point mutations were found, which were localized predominantly at the distal, globular part of the HA molecule. The mechanism by which these point mutations increased HA cleavability has not been defined. In the CEC-derived variants besides point mutations, an insertion of 54 nucleotides adjacent to the cleavage site was observed, which corresponds in its sequence to a region in the 28 S ribosomal RNA. This insertion is probably responsible for the altered cleavability of the CEC variants' HA, leading to increased growth potential and pathogenicity.