Rome Mice, selected for high (H) and low (L) antibody responses to xenogeneic erythrocytes (Biozzi mice, selection I), have been tested for their immune responsiveness to lysozymes. The anti-ring-necked pheasant egg-white lysozyme (REL) and anti-hen egg-white lysozyme (HEL) primary plaque-forming cell responses are 20 to 40-fold lower in L than in H mice. The heterogeneity of anti-HEL antibodies in the secondary response, as analyzed by isoelectric focusing, is highly reduced in L as compared to H mice. The in vitro anti-HEL plaque-forming cell secondary response of (H X L)Fl lymph node (LN) cells is induced by HEL-pulsed macrophages (M@) from H, but not from L, mice. Therefore, genetic differences affecting antibody responsiveness in H and L mice are expressed at the level of antigen-presenting cells. This genetic defect observed in L cells has been studied in more detail using an antigen-specific, T celldependent LN cell proliferative assay. After HEL-complete Freund's adjuvant (CFA) priming, LN cells from H or (H X L)Fl mice respond to HEL, REL (which is cross-reactive with HEL) and purified protein derivative of tuberculin (PPD), whereas LN cells from L mice do not respond to HEL or REL but proliferate in the presence of PPD. Proliferation of HEL-CFA primed (H X L)F1 LN cells is induced by HEL-or REL-pulsed M@ from H but not from L mice, whereas PPD-pulsed M@ from H or L mice give similar proliferative responses. By increasing %-fold the concentration of HEL used to pulse M@ from H and L mice, a comparable proliferative response is obtained when HEL-CFA-primed (H X L)Fl LN cells are stimulated with HEL-pulsed M@ from either line. This finding indicates that the genetic defect at the antigen-presenting cell level in L mice is not absolute, but rather reflects a different antigen handling by M@ in the two lines.