An in vitro lymphocyte-mediated cytotoxicity assay using [3H]proline-labeled target cells is described. The assay, modified from an original procedure of Bean et al., assesses the release of [3-H]proline by filtering the total culture fluid containing both trypsinized tumor cells and effector cells. Filtration is performed with a semiautomatic harvesting device using low suction pressure and large-diameter glass filters. Pretreatment of filters with whole serum diminishes adsorption of cell-free radioactive material considerably and thus increases the sensitivity of the assay. Nearly 100% of the radioactivity could be recovered with this harvesting device. The technique allowed the detection of cytolytic activities of lymphocytes after 6 h of incubation. Lymphocytes from patients with primary malignant melanoma showed a significantly higher cytolytic reactivity (P less than 0.001) than normal donors' lymphocytes against three different melanoma cell lines. In a series of parallel experiments on 36 patients and 18 normal donors, this modification of the [3H]proline test was compared with three different assays: the conventional microcytotoxicity test of Takasugi and Klein, the original [3H]proline microcytotoxicity test of Bean et al., and the viability count of tumor cells.