The AF6-78 antibody reacts specifically with mouse IgM of Igh-C[b] and related haplotypes (e.g., C57BL/6, C57BL/10, SJL, AKR, NZB). It does not react with IgM of Igh-C[a] or related haplotypes (e.g., BALB/c, C58, CBA, C3H/Bi, C3H/He, DBA/1, DBA/2, PL, RIII). Cross-reaction with IgM of Igh-C[e] haplotype (e.g., A/J) has been observed. AF6-78 antibody does not react with free μ heavy chain in vitro or in the cytoplasm of pre-B lymphocytes, which lack Ig light chain. It has not been shown to stimulate B-cell proliferation.