To determine whether a simple immunoperoxidase (IP) technique could replace the traditional immunofluorescence (IF) technique (by fluorescence microscopy) for determination of phenotype and monoclonality in cell suspensions from patients with lymphoproliferative disorders, we tested samples from 48 cases by both methods, using small panels of antibodies designed to determine proportions of B-cells, T-cells and cells bearing immunoglobulin light chains. There were 27 cases of chronic lymphocytic leukaemia (CLL), 6 of prolymphocytic leukaemia (ProLL), and 15 of non-Hodgkin's lymphoma (NHL). We found that IP was at least as reliable as IF in all cases. In the 47 B-cell cases, the mean proportion of cells apparently belonging to the malignant clone was higher by IP than by IF; conversely, the proportion of B-cells displaying the non-malignant light chain marker and the proportion of residual T-cells was apparently lower by IP. We conclude that IP is a useful and reliable technique for determining phenotype and monoclonality in lymphoproliferative disorders; because it is tolerant of delays to samples in transit and requires only simple facilities, it may be particularly suitable for small laboratories.