The mutagenic activity of paracetamol (PC) was studied on a group of healthy persons (3 men, 8 women) after the simultaneous administration of 3 X 1000 mg of PC and an identical dose of PC + 1000 mg of ascorbic acid within a period of 8 hours. Blood sample tests were made at intervals of 0, 24, 72 and 168 hours. A cytogenetic analysis showed that in 24 hours PC had increased the aberrant cell frequency to 2.77 +/- 0.37 per cent as distinct from 1.68 +/- 0.30 per cent (p less than 0.05) prior to the drug. In 72 hours, the rate of buccal mucosa micronuclei had risen to 0.38 +/- 0.07% against 0.19 +/- 0.06% prior to PC (p less than 0.01). Unscheduled DNA synthesis in peripheral lymphocytes had dropped to T/C = 2.06 +/- 0.54 (p less than 0.01) against 3.16 +/- 0.84 before PC. The lipid peroxidation level in the plasma remained unchanged. Ascorbic acid administered simultaneously with PC had no effect on the changes observed. A further follow-up of other PC side effects is recommended.