These studies further delineate the requirements for the establishment and transfer of SGVHD. We show that (a) two mechanisms distinguishable by radiation and drug sensitivities exist, (b) lethal irradiation correlates with a 100% incidence in the induction of SGVHD, whereas (c) both sublethal or lethal irradiation and cytoxan therapy are effective in ablating the host autoregulatory system in order to transfer autoreactivity, (d) unfractionated as well as nylon wool-nonadherent splenocytes effectively inhibit the transfer of autoimmunity, and (e) OX19 depletion of that population, however, destroys the autoregulatory effect present in normal splenocytes. To demonstrate complete inhibition of immune reactivity, twice the number of unfractionated splenocytes from normal animals was required for every splenocyte from autoimmune donors. Last, the infusion of effector splenocytes on 4, 7, and 14 d after transplantation correlates to a decrease from 100%, 70 to 0% incidence of SGVHD, thus emulating the incidence obtained in a pretransplant rat within 2 wk. These findings further clarify the immunobiological complexity of SGVHD and suggest that since autoregulatory cells already exist in normal animals that CsA-induced autoimmunity is a reflection of not an induced reactivity specific to one therapeutic reagent but the uncoupling of normal immunologic mechanisms essential in controlling autoimmunity.