In the present study, we extended the analysis of the regulation of inflammatory lymphokine production in mice with schistosomiasis mansoni. Splenic lymphocytes of chronically infected mice were briefly pulsed in vitro by soluble egg antigens, washed, and then cultured overnight. The supernatant culture fluid added to cultures of splenic cells of acutely infected or peritoneal lymphocytes of antigen-sensitized mice inhibited the production of migration inhibition factor (MIF). Elaboration of MIF suppressor factor (MIF-SF) required the Lyt-1-,2+,3+ subset of T lymphocytes. MIF-SF acted only on egg antigen-primed cells and required H-2 compatibility with the target cell for its suppressive effect. Further analysis with recombinant strains revealed that the factor interacted with I-AB or I-C subregion-compatible target cells. Experiments using immunoadsorbent columns with bound anti-I subregion alloantisera indicated that MIF-SF contained I-C subregion-encoded determinants. Extrapolation of this in vitro model to in vivo conditions would indicate that the granulomatous response is modulated by I region-derived suppressor factor(s) that regulate lymphokine production by TDH effector cells.