The synthesis and secretion of immunoglobulin M (IgM), as well as the relative ratio of membrane and secretory mu heavy chain (mu m and mu s, respectively), were evaluated in mycoplasma-contaminated B lymphoblastoid cell lines. The ratio of mu m to mu s was drastically lowered in infected cultures, and mu s chains could now combine with light chains. A 50 to 100% increase in IgM synthesis occurred in contaminated cultures, and small amounts of IgM were detectable in the culture media. These molecules possessed mu chains typical of secreted IgM. Reexpression of mu on the surface of B lymphoblastoid cells was substantially delayed in mycoplasma-contaminated cultures. Thus, mycoplasma contamination alters the synthesis and expression of a specific differentiated gene product (immunoglobulin) in B cell lines; such changes could significantly affect the interpretation of data on immunoglobulin synthesis by B cells with different phenotypes. This system may also provide a means of studying how mycoplasma infection alters specific gene expression in B cell lines.