Cell populations from twenty-two cases of B-cell neoplasia reflecting various stages of B-lymphocyte maturation were investigated for their ability to synthesise immunoglobulin (Ig) in vitro. All surface Ig positive neoplasms studied synthesised labelled Ig of the same light chain class as that expressed at the cell surface. Immature B-cell neoplasms, chronic lymphocytic leukaemia (CLL) and CLL-type lymphoma, synthesised only a minor proportion of their total protein as Ig; labelled free light chain was the only detectable secreted Ig product in thirteen cases and was in excess in the remaining four. Labelled heavy chain was detected in cell lysates in all but one case. Follicular centre cell lymphomas, neoplasms of more mature B-lymphocyte types, synthesised more of their total protein as Ig and showed a more balanced synthesis of heavy and light chains compared to the other neoplasms studies. The Ig synthesis patterns and, in particular, free light chain production, are discussed in relation to normal B-lymphocyte maturation.