The most profound toxic effects of many physical and chemical agents are observed when exposure occurs during mammalian development. The development of immunocompetence in mammals involves a precise sequence of steps which begins during gestation and is completed during postnatal life. Perturbation or abrogation of the developmental sequence of events may lead to life threatening dysfunctions as exemplified by certain heritable defects in immune system development. The authors have examined the effects that prenatal and/or early postnatal exposure to a variety of known or suspected physical and chemical immunomodulators may have on immune function in young adult rodents. Mice and/or rats were exposed in utero and/or during early postnatal life to the following agents: cyclophosphamide, nonionizing radiation, cyclosporin A, urethan, di-n-octyltin dichloride or NiCl2. At immunological maturity animals exposed to these agents were assessed for immune system functional integrity.