An investigation has been made into the ability of human neoplastic B lymphocytes expressing surface IgM and IgD to export IgD in culture. Cells that expressed surface Ig of the lambda light chain type frequently exported IgD (10/12 patients), whereas cells expressing surface Ig of the kappa light chain type exported no IgD, although most (8/11 patients) were able to export IgM. It appears, therefore, that in most of the 23 cases studied, cells synthesizing IgD with lambda light chains can both express and export IgD, whereas those synthesizing IgD kappa can only insert it into the surface membrane. This finding and the known preponderance of lambda in plasma IgD imply that the possession of a lambda chain facilitates the IgD secretory pathway, a conclusion that implicates a control mechanism subsequent to the surface/secretory dichotomy arising from different splicings of heavy chain messenger RNA.