The clinical reaction and the immunological response to influenza virus vaccine were studied in 43 B-cell chronic lymphocytic leukaemia patients. The Vaxigrip vaccine was administered containing the antigens A/Ghizhou/54/89, A/Singapore/6/86, and B/Yamagata/16/88. The side-effects observed were minimal and well tolerated. Antibody production with titres > 1:20 on day 15 was observed at least for one antigen in 35 patients (81%). In 23 of them (63%) this response was retained on days 30 and 60. Patients with IgG levels ( < 700 mg/dl) responded lesswell as compared to those having normal IgG levels ( > 700 mg/dl).