A panel of 73 samples, including 52 primary breast carcinomas, 10 normal breast tissues and 11 axillary lymph nodes, has been analysed for the presence of amplifications and gross structural alterations, in the oncogenes c-erbB-2, c-erbA, c-myc, N-myc, c-mos and c-Ha-ras. The tumours were also classified, graded and staged histopathologically and their DNA ploidy (42 samples) was determined by flow cytometry. Three breast cancer cell lines (MCF7, ZR-75-1 and T47D) were also included in the study. Amplification of c-erbB-2 was detected in 28% of the tumours, of which 91% had an increased steady-state level of c-erbB-2 mRNA. Amplification of c-erbA was found in 23% of tumours and was always associated with the amplification of c-erbB-2. Ten out of 12 (83%) tumours which had c-erbB-2 and c-erbA co-amplification had metastasised to axillary lymph nodes (P less than 0.006). However, the human thymidine kinase gene, which is present at the same chromosomal location as these two oncogenes (17q21-22), was amplified in only tw tumours. Amplification of c-myc was detected in 21% of the tumours studied, of which 82% (P less than 0.005) were of histopathological grade 3 and none were of grade 1. Flow cytometry showed that 90% (P less than 0.01) of the analysed tumours with c-erbB-2 and c-erbA co-amplification, and 70% (P less than 0.1) of those with c-myc amplification were DNA aneuploid. This study demonstrates the potential value of c-myc amplification in the assessment of the tumour grade, rather than metastatic potential; and of the co-amplification of c-erbB-2 and c-erbA as a strong indicator of metastatic potential, rather than tumour grade.