It has been shown that a mutation occurred in C3H/HeJ mice rendering the B lymphocytes of those animals defective in their response to LPS. Further studies suggested that in C3H/HeJ mice, macrophages also had defective biological responses directly related to LPS; moreover, C3H/HeJ macrophages showed failure to be activated by lymphokines to kill tumor cells in vitro. In an attempt to determine whether the defect(s) of C3H/HeJ macrophages could also affect their response to other lymphokines, we compared the responsiveness of mineral oil-induced peritoneal cells (PEC) from C3H/HeN and C3H/HeJ mice to migration inhibition factor (MIF). Supernatants of antigen-stimulated immune spleen cells were tested against PEC, employing the indirect agarose microdroplet technique. No migration inhibition with PEC from C3H/HeJ mice was detected, whereas PEC from C3H/HeN mice were significantly inhibited by even a 1/32 dilution of MIF supernatant. However, responsiveness to MIF of C3H/HeJ PEC could be induced. In fact, in vivo intraperitoneal injections of Mycobacterium bovis, strain BCG, seven days before the in vitro assay could render the C3H/HeJ PEC responsive to MIF. The usual lack of responsiveness to MIF by C3H/HeJ macrophages seemed to be related to some form of suppression, since mixture of PEC from C3H/HeN mice with 10% PEC from C3H/HeJ mice resulted in undetectable migration inhibition at any of the MIF dilutions employed. The degree of suppression of the responsiveness of MIF appeared to be related to the percentage of C3H/HeJ PEC added, with 5% C3H/HeJ PEC partially suppressing the C3H/HeN PEC response, and 2.5% C3H/HeJ having only borderline effects. We found that T cells did not appear to be involved in the suppression, since pretreatments with anti-theta serum plus complement of PEC of both the C3H substrains affected neither the defective responsiveness to MIF of C3H/HeJ PEC nor the suppression caused by these PEC. The percentage of PEC phagocytizing latex particles after injection with mineral oil was 60-70% for the C3H/HeN mice and only 30-40% for the C3H/HeJ mice, whereas no differences have been found when the phagocytosis of 51Cr-antibody coated erythrocytes was analyzed. These differences in PEC subpopulations did not seem to be reflected by the pattern of cell migration in the absence of MIF, since this was similar for C3H/HeN and C3H/HeJ PEC. Further studies are in progress in an attempt to elucidate the mechanism(s) of action responsible for the defective responsiveness to MIF of PEC from C3H/HeJ mice and for their suppressive activity on C3H/HeN PEC.