Serum IgA levels of 35 healthy IgA-deficient index cases, of their 180 first-degree relatives, and of 31 other family members were studied. IgA deficiency was detected in 7.2% of the first-degree relatives, which is significantly more than the 0.25% frequency of IgA deficiency in healthy Finnish blood donors. Subnormal serum IgA levels were found in additional 13 (7.2%) first-degree relatives. The familial clustering of IgA deficiency seemed to be controlled by multigenic factors. IgA-deficient persons and their family members were tested for antinuclear and antithyroid antibodies, for rheumatoid factor (by latex test) and for other anti-IgG antibodies (by Ripley test). Antibodies against bovine milk were also studied. The relatives of IgA-deficient persons did not have more of these antibodies than the controls. It is suggested that various serological abnormalities are a consequence of the IgA deficiency rather than of genetic defects per se.