Recent experimental findings indicate that individual B cells can produce simultaneously heterogeneous Ig molecules. Three orders of heterogeneity have been described. (1) In two published cases of human myeloma, individual malignant cells produced Igkappa and Iglambda (case Im) or IgMlambda and IgGkappa (case Br). (2) Immunofluorescence studies repeatedly showed that normal B cells may bear surface Ig of two different classes or cytoplasmic Ig of one class and surface Ig of another class. However, simultaneous production of two different CH chains by immunized B cells or myeloma cells appeared to be rather rare (1-3%). (3) Immunization of mice with two different antigens regularly results in the appearance of cells (1-3%) simultaneously reacting with both antigens. Specific inhibition of the one activity does not interfere with the other. Furthermore, bispecific cells cultured individually generate daughter cells, each producing antibody of only the one or the other specificity. It can be inferred from these findings that early B lymphocytes are pluripotential and that differentiation involves a process of phenotypical restriction directing the cell (plasma cell) to produce high quantities of one type of Ig molecule.