An initiation study of mineral oil-induced plasmacytoma (MOPC) 315 heavy chain immunoglobulin (H315) in vitro has been conducted using formyl-[35S]methionyl-tRNAfMet and a highly purified 18 S message from MOPC 315 solid tumor in a crude rabbit reticulocyte lysate system. The product was specifically precipitated by antibodies directed against MOPC 315 immunoglobulin and H315. The in vitro H315 products terminally labeled with formyl-[35S]methionine or internally labeled with [3H]leucine were electrophoretically identical with in vivo H315 on sodium dodecyl sulfate-polyacrylamide gels. All of the [35S]-methionine was incorporated at the NH2 terminus, not internally, since there is a near complete recovery of [35S]methionine following one cycle of Edman degradation. The NH2-terminal cyanogen bromide peptide, CN2, of in vivo and in vitro H315 co-migrated exactly on gel electrophoresis under conditions which completely resolved two proteins differing in size by only 14 amino acids. These data strongly suggest that there is no NH2-terminal precursor of H315 in this system. Cyanogen bromide peptide profiles of in vivo and in vitro H315 were chromatographically indistinguishable. Three peptides, CN1, CN2, and CN4, which represent approximately 85% of the total amino acids of H315 were isolated and further characterized by electrophoresis and paper chromatography. All were very similar to the corresponding peptides of authentic H315. We conclude that the fidelity of H315 translation is preserved in vitro.