Sera from 244 patients with haematological malignancies were examined for beta 2-microglobulin (beta 2m) levels. There were 142 leukaemias, 32 malignant lymphomas, three immunoblastic lymphomas, two pseudolymphomas and 65 multiple myelomas. Culture supernatants from various established cell lines were also tested. The phenotype facilitating beta 2m shedding from the cell surface appeared to be independent of the specific IgG heavy chain allotypes; however, a myeloma group with normal serum beta 2m levels showed a significant association with the specific Gm allotypes. The determination of serum beta 2m levels can provide valuable information on the proliferative stage of the disorders, the effectiveness of chemotherapy, and be a diagnostic aid for blastic crisis in chronic myelocytic leukaemias, and for subtyping lymphoid malignancies.