A series of adult soft-tissue tumours were screened for the presence of activated oncogenes by transfecting tumour DNA into NIH3T3 mouse fibroblasts. In these studies an activated K-ras gene that contained a mutation at the second position of codon 12 (GGT----GAT) was found in a leiomyosarcoma. In addition, following transfection of DNA from a liposarcoma, we identified an activated gene that failed to hybridize to probes prepared from 10 known human oncogenes (K-ras, H-ras, N-ras, ret, met, trk, mas, dbl, raf and hst) that have previously been detected in DNA transfection experiments. BamHI-BamHI fragments of this activated gene of 3.5 and 8.5 kb were cloned from NIH3T3 secondary transfectants using a probe that detects the human alu family of highly repetitive DNA sequences. Repeat-free subclones of these BamHI fragments were used to map this gene to human chromosome 19 (p13.2-q13.3). Our studies also demonstrate that a subclone from one of the BamHI fragments detects a 3.0 kb transcript in primary and secondary transfectants.