SUMMARYThe mobilities of murine splenocyte surface immunoglobulin heavy chains were compared by polyacrylamide gel electrophoresis in SDS‐containing buffers, with those of the heavy chains of human IgM, IgG, and IgD, and toad IgY. Human δ‐chain showed a mobility only slightly faster than that of human μ‐chain, and required double‐labelling techniques for unequivocal resolution. The mobility of the slower murine splenocyte surface heavy chain was identical to that of human μ‐chain, but that of the second splenocyte heavy chain was significantly faster than that of human δ‐chain, and similar to that of the heavy chain of toad IgY. Although the second murine splenocyte heavy chain is clearly distinct from μ‐chain, no physico‐chemical evidence exists which relates it directly to human δ‐chain.