Ataxia-telangiectasia (AT) is a genetic disorder of unknown pathogenesis, with primary effects on the immune and nervous systems. The presence of a fetal-like thymus and elevated alpha-fetoprotein (alpha FP) levels in patients with AT suggests that suppressed mesodermal development may be a factor in the development of this disease. We investigated this hypothesis by using electrophoretic and quantitative analyses to test for the presence of other fetal proteins in mesodermal tissues. With the exceptions of alpha FP and carcinoembryonic antigen, all other proteins assessed in these patients were present at levels or in isozymic patterns characteristic of a normal, nonfetal state.