Anti-proliferative activity was investigated of C18:2 9cis,11trans (conjugated linoleic acid – CLA) isomer isolated from sheep milk fat. The research was conducted on sheep milk fatty acids in natural composition (FA-1) containing 2.0% of the C18:2 9cis,11trans CLA isomer and preparation enriched with CLA, containing up to 9.2% of the CLA isomer (FA-2). The process of enrichment of FA-1 was carried out in two steps. Step I was to remove mid-chain-length saturated fatty acids, primarily C16,by forming adducts with urea (to obtain FA-U), while in step II the short-chain acids were removed by way of supercritical extraction with CO2. The final preparation obtained (FA-2) contained 9.2% of the C18:2 9cis,11trans isomer. A natural sheep milk fatty acid composition (FA-1), the CLA-enriched product (FA-2) and a commercial CLA preparation (CP) were tested for anti-proliferative activity against the cells of human cervical carcinoma (KB), and leukaemia (HL-60). The ID50 dose (resulting in a 50% inhibition of cell proliferation) of FA-2 was found lower than that of the CP preparation. The SRB test showed.