The mRNA isolated from B lymphocyte tumor cell lines directs synthesis of two forms of p heavy chain, one with a molecular weight of 67K and one of 64K. When these cell lines are converted to IgM-secreting cells by fusion with a myeloma cell, the 64K form of c predominates; thus it is designated ps (p-secreted). The 67K form correlates with the presence of surface IgM; thus it is designated p,,, b-membrane). Cells that make both forms of p chain have two mRNAs, one of 2.4 kb that encodes ps and one of 2.7 kb that encodes pm. The difference between the cs and pm mRNAs can be localized to their 3’ends by hybridizing 32P-cDNA copies of the mRNA to a cloned copy of ps mRNA, treating the mixtures with SI nuclease, and resolving the nuclease-resistant duplexes by electrophoresis. By probing the separated species of RNA with a DNA copy of the 3’untranslated region of ps mRNA, it was shown that the 3’ends of the two p mRNAs do not cross-hybridize. The difference between the two RNAs was mapped to the 3’edge of the Cp4 domain. Apparently two separate 3’terminal sequences for p mRNA are encoded in the genome, one that specifies an amino acid sequence appropriate for membrane-binding and a second that is involved in secretion. At different stages of immunocyte development, different p mRNAs predominate: p,,, during the lymphocyte stages and p= during the secretion stages.