ABSTRACT:The development of immunocompetence during gestation depends upon the sequential differentiation of antigen‐specific lymphoid cells in the context of epithelial inducing microenvironments. These early intrauterine events, which appear to be antigen‐independent, include clonal diversification of idiotypes and isotypes as well as commitment to B or T cell lineages. The steps in cellular maturation can be traced through the use of lymphocyte differentiation markers. Cooperation among lymphoid subsets, as well as from nonlymphoid cells and possibly other cofactors, is necessary for the effective function of this array of lymphocytes. The rate of expansion of functional immunity may be limited as much by the ontogeny of these collaborating influences as by the intrinsic immaturity of the B and T cells themsel