Virus isolation or influenza virus antigen detection are the most rapid tests for diagnosis in the acute stage of influenza virus infection. As serology is easier to carry out, the synthesis of serum IgM, IgA and IgG was studied in two well-defined patient groups, infected with influenza B virus (cohort 1, n = 37) and influenza A virus (cohort 2, n = 40), diagnosed by antigen detection and/or virus isolation within 36 h after onset of symptoms. IgM was found in 13 influenza B patients (35%), IgA in 12 patients (32%), whereas a significant antibody rise was found in 33 patients (92%) by enzyme-linked immunosorbent assay (ELISA) and 74% by haemagglutination inhibition assay (HAI). For the influenza A cohort these numbers were respectively 18 (45%), 27 (68%) and 24 (62%) HAI (72%). In age-matched controls, who were bled on the first day of illness of the enrolled patient low prevalence was found for IgA and IgG, for influenza B respectively in 2 and 18%, and for influenza A in 4 and 39%. Studying the kinetics of the antibody response, we found that virus specific IgA and the bulk of IgG is synthesised within the first week of the infection. It is concluded that the finding of a specific serum IgA is highly indicative of an acute influenza infection.