A new methodology is reported determining the cytogenetic damage in stimulated human peripheral lymphocytes. The methodology is based on the investigation of the frequency of cells with micronuclei utilizing cytochalasin B. The spontaneous frequency of cells with micronuclei in the group of healthy volunteers (9 females and 3 males) of the mean age of 37.8 years was 18.42 micronuclei/1000 cells. Furthermore, the action of the analgesic agent paracetamol on the frequency of cells with micronuclei was examined; after the dose of 3 X 1000 mg of paracetamol administered within 8 h no increase in the occurrence of micronuclei within 1 week was found. The advantage of the use of the test for the determination of micronuclei with cytochalasin B in the testing of new drugs are discussed.