Investigations included blood donors for extracorporeal circulation, as well as patients (12) before and after cardiopulmonary by-pass. CMV infection is a frequent consequence of multiple fresh blood transfusions in persons with aortocoronary "by-pass" in 25% operated patients. If a patient receives more than 10 transfusions of fresh blood, frequency of serologically positive reactions to CMV is found in approximately 40-60% operated patients. Investigations included 168 healthy individuals, voluntary blood donors, 18-30 years old, both male and female, from various backgrounds and with various occupations. It was found out that amount of antibodies against CMV for men was 42,1% in spring and 66,7% in winter, while the amount of antibodies against CMV for women was 70% in spring and 81% in winter. Complement fixation reaction method was applied.