The cytotoxic reactivity of cells recovered from host organs undergoing severe graft-versus-host (GVH) reactivity resulting from donor-recipient histoincompatibility at the entire H-2 complex, the H-2 I region alone, or the H-2 K/D regions have been examined. In all H-2 or H-2 I-region-disparate combinations acute lethal GVH disease occurred. In H-2-K/D-region-disparate combinations mortality was only 60-80%; however, injections of Interleukin-2 increased mortality to 100%. Donor antihost cytotoxic lymphocytes (CTLs) could be recovered from the organs and tissues of GVH animals mismatched at the entire H-2 complex or K/D mismatches but not from animals mismatched at only the I regions. In K/D region mismatches, only a weak transient antihost CTL response was detected. In the I region mismatches, antidonor cytotoxic reactivity appeared most frequently and could be expanded in vitro using Interleukin-2 (IL-2). In addition, lethal GVH disease was induced in hosts with cloned anti-I-A, Lyt-1+ noncytotoxic T helper cells. Thus, if CTLs or T suppressor cells are essential for the development of lethal GVH disease, they must be derived from the host per se. We conclude, therefore, that there is no absolute correlation between lethal GVH and the development of donor-derived CTLs or T suppressor cells.