We have used a 4-day MTT colorimetric assay to study drug sensitivity of leucocytes from leukaemia patients and from normal donors. Response to Adriamycin, vincristine, aclacinomycin A, 3'-deamino-3'-morpholino-13-deoxo-10-hydroxycarminomycin (MX2), and melphalan has been determined, together with the effects of the resistance modifiers verapamil, cyclosporin A, and ethacrynic acid. Sensitivity of chronic lymphoblastic leukemia (CLL) lymphocytes to vincristine was much greater than that of normal lymphocytes or of leucocytes from myeloid leukaemia patients. These cells were also more sensitive to melphalan. Verapamil and cyclosporin A at clinically achievable doses of 1 microgram/ml produced significant chemosensitisation in normal and leukaemic specimens, but the sensitisation ratio was greater than or equal to 2 only in a minority of specimens, except in the case of sensitisation to vincristine seen in the majority of CLL specimens. Sensitisation was generally greater in the more chemo-resistant specimens. The ratio of sensitivities of cells to Adriamycin compared with aclacinomycin A was greatest in the more Adriamycin-resistant specimens which supports the idea that cross-resistance between these agents may not be great. This was not, however, true for the ratio of Adriamycin/MX2 sensitivity. Use of the MTT assay may allow the identification of patients who would benefit from treatment with resistance modifiers or with 'low-resistance' anthracyclines.