The DS-1 antibody reacts specifically with mouse IgM of Igh-C[a] and related haplotypes (e.g., BALB/c, C58, CBA, C3H/Bi, C3H/He, DBA/1, DBA/2). It does not react with IgM of Igh-C[b], Igh-C[e], or related hapltypes (e.g., C57BL, SJL, A, AKR, NZB). DS-1 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.