Immunoglobulin synthesis has been investigated in 55 cases of B lymphocytic disease. Free light chain was the predominant product of Ig synthesis and the sole secreted Ig product from neoplasms lacking surface Ig or expressing sparse surface IgM as detected by fluorescent-labeled antibody. In contrast, neoplastic cells expressing strong surface IgM or IgG synthesized and secreted balanced amounts of heavy and light chain. Intermediate patterns of synthesis were shown by neoplastic cells expressing IgM with IgD. These patterns of synthesis occurred irrespective of the disease entity and are discussed in relation to models of B cell maturation.