The activity of O6-methylguanine-DNA methyltransferase was determined in fibroblast cultures from 45 patients with lung cancer, 39 patients with cutaneous malignant melanoma, and 29 healthy controls. This enzyme is a critical parameter for the capacity to repair O6-methylguanine (O6-mGua) adducts in DNA, and a decreased activity might therefore be responsible for an enhanced susceptibility to cancer. The assay was performed with 8 x 10(6) fibroblasts which were homogenized and incubated with a known amount of O6-mGua containing DNA. The remaining substrate was determined fluorimetrically after high performance liquid chromatographic separation. O6-mGua repair was significantly reduced in lung cancer patients [6.64 +/- 4.32 (SD) pmol O6-methylguanine repaired/8 x 10(6) cells] as compared to healthy controls [10.35 +/- 5.42, P less than 0.0022] or patients with cutaneous malignant melanoma [10.83 +/- 6.66]. The lowest mean values were detected in a subgroup of 16 lung cancer patients with a tumor manifestation below 46 years of age (5.06 +/- 3.89). Fibroblasts from 4 patients with lung cancer had no detectable repair. We conclude that a reduced capacity to remove O6-mGua adducts may represent a further mechanism of individually enhanced lung cancer risk.