Amplification of the int-2 oncogene was observed in 10 out of 78 (13%) samples of DNA isolated from cells of primary breast tumor. A clinico-morphological examination established a correlation between this molecular-genetical anomaly and tumor size: gene amplification was detected in 1 out 34 (3%) cases of breast tumor under 2 cm and in 9 out 44 (20%) tumors measuring 2 cm and more (p < 0.05). Similarly, a kind of correlation was found between gene amplification and low differentiation of breast cancer cells: enhanced copying ability of the oncogene was registered in 2 out 4 (50%) breast tumors with low cell differentiation, 8 out 69 (12%) tumors with medium cell differentiation, while it was not identified in 5 breast carcinomas with high cell differentiation (p < 0,2).