The affinity of anti-ldl autoantibodies to ldl fraction isolated from healthy donors was five-fold lower that affinity to ldl isolated from atherosclerotic patients. We have tested numerous types of ldl modification and found out that antidodies have the highest affinity to desialylated ldl particles. Affinity of anti-ldl antibodies was rather high to mda-modified ldl, too. This ldl modification was comparable to total ldl fraction isolated from atherosclerotic patients. However mda-modified ldl has lower affinity to antibodies as compared to disialylated ldl. Other tested ldl modifications demonstrated low affinity to ldl. We believe that appearance of anti-ldl antibodies in the blood is a result of ldl desialylation probably due to trans-sialidase activity.