The proportions of six immunoglobulin isotypes (IgA, IgM, IgG1, IgG2, IgG3, and IgG4) in rubella antibody responses were quantified in 40 serum samples (20 patients). The first sample from each patient was taken during the first days of the illness, and the second sample 10 +/- 1 days later. A tenfold average increase in antibody concentration was observed between the first and the second sample. IgM was the predominant isotype in the first sample (average, 73% of all antibodies), followed by IgG1 (19%). IgA and IgG3 antibodies were detected in only a few of the first samples, and IgG2 or IgG4 in none. In the second samples IgG1 was the predominant antibody isotype (average, 59%). Next came IgM (23%), followed by IgA (8%) and IgG3 (3%). No IgG2 or IgG4 antibodies were detected. Although the proportion of IgM antibodies was lower in the second than in the first samples, their concentration increased in all patients (the average factor was 7). The kinetics of the IgA response was irregular. In some patients there was a strong (up to 90-fold) increase in IgA antibodies, but in two patients a small drop was detected. The kappa- to lambda-chain ratio of rubella antibodies appears to be close to the expected 2:1. It decreased in some patients during the 10 days and increased in others.