The proportion of foetal thymus lymphocytes (FTL) that binds the bacterial antigens beta-galactosidase and flagellin is high in early foetal life. Binding of beta-galactosidase, and the response by FTL in mixed lymphocyte culture falls during gestation. Some FTL bound both antigens, suggesting that immature lymphocytes are not fully restricted in their capacity to recognize antigens. Such findings have been reported in foetal lymphocytes from other species. We suggest that cellular diversity may partly be generated by progressive restriction of antigen recognition by individual lymphocytes, which may result from progressive stabilization of genetic repression during lymphocyte multiplication.