A solid-phase immunoassay for terminal deoxynucleotidyl transferase has been developed using a primary antibody-coated polystyrene bead and secondary antibody conjugated with horseradish peroxidase. The immunoassay was compared with assays for enzyme activity and detection of antigen with immunofluorescence using cells from peripheral blood and bone marrow from patients with leukemia or lymphoma. In each instance, the solid-phase immunoassay correlated correctly with cellular samples judged to be positive by other tests. However, the level of detection of terminal transferase antigen in plasma or serum of patients with leukemia did not reflect accurately the level of terminal transferase in neoplastic cells. The solid-phase immunoassay was greater than 100-fold more sensitive than conventional assays for enzyme activity, rendering it potentially useful for quantitatively monitoring terminal transferase in patients with leukemia.