Precursors of antibody-producing cells in the mouse are functional homologues of bursa-derived lymphocytes in birds (1); they have antibody-like immunoglobulin receptors (2-8) specific for limited antigenic moieties (6-16) on their cell membrane which are largely of the 3,M immunoglobulin (Ig) 1 class in experimentally virgin animals of this species (16,17). Combination of antigen with the specific receptor and the cooperation of specific thymus-derived lymphocytes triggers division and differentiation of these precursor cells into mature antibody-secreting cells (6,(18)(19)(20)(21)(22)(23).In the preceding paper (24), we demonstrated that antibody to mouse /z-chain specifically suppressed primary plaque-forming cell (PFC) responses of all Ig classes in mouse spleen cell cultures stimulated with heterologous erythrocytes. This suppressive effect was mediated through the precursors of antibody-producing cells and appeared to involve, for the most part, a potentially reversible saturation of antigen receptors with/z-chain determinants by the anti-/z and prevention of antigenic stimulation rather than elimination of these virgin precursor cells with surface /z-chains via cell death. Antibodies to mouse 3`1 and 3'2 heavy chains suppressed both 71 and 3'2, but not 3"M or 3`A primary PFC responses.