Serum levels of the four main immunoglobulins, IgA, IgM, IgG and IgD were estimated in a sample of 54 subjects with tuberous sclerosis and the data compared with similar findings in a sample of 100 mentally retarded subjects chosen at random. Normal levels of IgA and IgD were found in the T.S. group, but elevated levels of IgM significant at P equals 0.001 was found in 49 of the 54 cases. A reduction in IgG levels (significant at P equals 0.001) was a much less constant feature being found in 29 of the 54 subjects. The significance of these results in the light of the observation that one of the principal causes of death in T.S. is from recurrent infection is discussed.