Influenza B viruses have been represented in human populations in recent years by two divergent strains, B/Victoria/2/87-like and B/Yamagata/16/88- like viruses.Drift in the B/Yamagata-like viruses has been identified by a panel of ferret antisera during the past year and has raised the question of need for further alterations in vaccine composition.In order to determine human responses to new B/Yamagata-like variants, sera from 23 children and 30 adults who were immunized with vaccines containing the antigens of influenza virus B/Yamagata/16/88 were tested.Against B/Hong Kong/22/89 (an immunologically distinguishable variant of B/Yamagata), post-immunization geometric mean titers (GMT) of neu-tralizing antibodies were only approximately half those against the vaccine virus (reciprocal GMT = 344 for B/Yamagata vs. 157 for B/Hong Kong). Among the subset of children who were immunologically unprimed the disparity was more marked (GMT = 473 for B/Yamagata vs. 141 for B/Hong Kong). Against a panel of variants of B/Yamagata-like viruses that have drifted even further immunologically than B/Hong Kong (Panama/45/90, Bangkok/163/90, Illinois 2005, and Texas VC-1 1037), adult postimmunization sera tested by hemagglutination inhibition (with detergent split virus as antigen) showed no difference in GMT (218 for B/Yamagata, 218 for B/Panama, 218 for B/Illinois, 230 for B/Texas, and 179 for B/Bangkok). These results suggest that adults would be adequately protected against the newly drifted influenza B strains related to B/Yamagata, but that children, particularly those with little or no previous exposure to influenza B antigens, might be suboptimal for protection against infection. The data suggest that vaccine strategies for adults may be inadequate for children.