The role of genetics and environmental factors determining serum concentrations of immunoglobulins (IgA, IgG, and IgM) were analyzed in 20 pairs of monozygotic (MZ) and 11 pairs of dizygotic (DZ) twins. Respiratory chemosensitivity to hypoxia and hypercapnia was also measured. Intrapair variances for α2-, β-, and γ-globulins, IgG, IgA, and IgM were significantly smaller than interpair variances in MZ, indicating that these variables are more similar between pairs than among pairs in MZ. In DZ, intrapair variance for IgM was significantly smaller than interpair variance. Intrapair variance in MZ for γ-globulin, IgG, IgA, hypoxic ventilatory response, and hypercapnic ventilatory response were significantly smaller than those in DZ, indicating that these variables are genetically controlled. Subjects with low IgA showed a blunted hypoxic ventilatory response, and subjects with high IgA, a vigorous ventilatory response. These results indicate that while IgM is controlled predominantly by environmental factors, IgA and IgG are genetically determined in young healthy subjects. Serum IgA levels and hypoxic ventilatory response are interrelated in young healthy subje