A rapid chemosensitivity assay was developed, employing the human continuous leukemic cell lines HL 60, K 562, FLG 29.1. This automated colorimetric assay is based on the characteristic of viable, metabolically active cells to cleave p-iodonitrotetrazolium violet (INT) into a red formazan derivative, whose optical density is readable at 492 nm by an automated microtiter-plate reader photometer. A linear relationship was found between the viable cell number and the optical density of INT cleaved by the cellular samples. Dead cells did not reduce INT and did not interfere with the formazan derivative generation and the photometric reading. Leukemic cell lines were also tested for INT formazan derivative generation after exposure to antileukemic drugs at various concentrations, representative of plasma levels obtainable in vivo. A dose-dependent inhibition was detected, with different sensitivity patterns, related both to the drugs and to the different cell lines. A significant correlation between the viable cell number and the amount of tetrazolium salt cleaved was also demonstrated after drug exposure. INT assay allows the processing of a great number of samples and gives the opportunity to screen several drugs, saving time and yielding fully reliable results.