Abnormalities of the proportions of peripheral blood lymphocyte subpopulations and of immunoglobulin serum levels were found in twenty patients affected by Turner's syndrome. A slight but significantly decreased percentage of circulating T and B cells, and increased percentage of null cells and a decreased in vitro responsiveness of lymphocytes to phytohaemagglutinin, concanavalin A and pokeweed mitogen were found in Turner's syndrome patients. IgG serum level was found significantly decreased in comparison with age-matched fifty-seven normal males and fifty-seven normal females and IgM serum level was intermediate between female and male values; Turner's syndrome patients with monosomy had an IgM serum concentration very close to male values. The derangement of T and B lymphocyte subpopulations, probably related to the aneuploidy, does not seem to be a severe one but it could account for the immunoglobulin abnormalities and for the association of Turner's syndrome with immunological disorders such as autoimmune diseases. The role of X chromosome on IgM serum level is discussed.