Immunoglobulin levels (IgG, IgA, IgM), percentage and number of circulating B and T lymphocytes, mitogen responsiveness (PHA and Con A) and activity of thymus-dependent serum factor (SF) were estimated in 29 patients with Turner's syndrome (TS), ranging in age from 2.2/12 to 18.4/12 yr. IgG and IgM were decreased in 14 and 12, respectively, while IgA was very low in 1, and increased in 7. Peripheral blood B and T lymphocytes, in vitro responsiveness to PHA, and Con A, and SF activity were in the normal range. The present immunological study suggests that TS is often associated with low levels of serum IgG and IgM. The possible mechanism of these derangements, whether genetic or hormonal (low estrogen/androgen ratio), is discussed.