The expression of membrane transferrin receptors (TfRs), as defined by monoclonal antibody OKT9, and the nuclear proliferation-associated antigen Ki-67 were examined in 159 cases of hematological malignancy. Of the "chronic" B and T cell leukemias studied (n = 85), 61% showed less than 5% OKT9-positive cells and only 7% of cases were TfR+ (defined as greater than 20% positive cells). For comparison, the acute leukemias (n = 62) showed higher (p less than 0.001) TfR expression with 39% TfR+ cases, although there was considerable variation within diagnostic subgroups. Nuclear Ki-67 expression was generally insignificant (less than 1%) in chronic leukemias (78 of 88 cases), although two of eight B cell-type prolymphocytic leukemia and four of four cases of plasma cell leukemia showed greater than 10% Ki-67+ components. In contrast, 47% (31 of 66) acute leukemias had greater than 10% Ki-67+ cells, although there appeared to be no relationship between Ki-67 expression and leukemic type. Combined assessments of TfR and Ki-67 expression revealed a Ki-67- TfR- phenotype in 82% of chronic leukemias, compared with 28% of acute type, and a Ki-67+ TfR+ pattern was found in 27% of acute proliferations but not in any case of chronic leukemia. The determination of membrane TfR expression appears to have little value in the diagnostic differentiation of leukemias, whereas Ki-67 is considered to be a useful supplementary investigation in defining high grade tumors, in the recognition of prognostically poor cases of otherwise well defined low grade malignancy, and of potential value in resolving discrepancies between morphological and immunophenotypic features in leukemias of "intermediate" type.