Six cases of influenza B occurred in transplanted patients in a period of 3 weeks. Three renal allograft recipients recovered within 5 days without antiviral therapy. Two allogeneic bone marrow recipients were treated with ribavirin inhalations during the leukopenic phase. Treatment was given until influenza B was no longer detected and fever disappeared after 5 and 6 days, respectively. Engraftment was not delayed and no side-effects were noted. One recipient of autologous marrow was treated for 2 days, but ribavirin was discontinued due to pleuritic pain. We conclude that influenza B can be spread by asymptomatic carriers in the nursing staff and in spite of reversed isolation with the use of gown, hand wash and gloves.