The anti-µ was specific for µ-chain determinants; its activity was dose dependent, but was not mediated by killing cells with surface µ-chain determinants. Free M but not G myeloma proteins in solution effectively competed with µ-bearing cells for the anti-µ. An excess of anti-µ was necessary in the cultures for 48 hr to insure complete suppression of 5-day responses. However, after removal of excess anti-µ at 48 hr, responses could be stimulated by newly added antigen in cultures where incubation was prolonged to 7 days. Anti-µ was most effective when added at the initiation of cultures and had no suppressive effect when added at 48 hr. Excess antigen did not effectively compete with anti-µ for antigen receptors. Precursors of antibody-forming cells were shown to be the cell population where the suppressive activity of anti-µ was mediated.

The experiments suggest that anti-µ combines with µ-chain determinants in antigen-specific receptors on the surfaces of antibody-forming cell precursors, prevents effective stimulation by antigen and subsequent antibody production. To explain suppression of responses in all Ig classes by anti-µ, several models were proposed. It is not possible to determine from the data whether stimulation of precursor cells with G or A receptors requires concommitant stimulation of separate cells with only M receptors, or whether cells bearing M receptors are precommitted to or differentiate into cells capable of synthesis of other Ig classes, or whether receptors of M and another Ig class are present on some virgin precursors or the second Ig receptor appears after antigenic stimulation.