Mutagenic activity of paracetamol (PC) was studied in a group of healthy persons (3 men, 8 women) after a simultaneous administration of 3 x 1000 mg of PC and of identical dose of PC + 1000 mg of ascorbic acid within an 8 hr. period. Blood sample tests were made at intervals 0, 24, 72 and 168 hrs. Cytogenetic analysis showed that in 24 hrs. the PC increases the aberrant cell frequency to the value of 2.77 +/- 0.37 percentage vs. 1.68 +/- 0.30% (p less than 0.05) prior to the administration of the drug. In 72 hrs., the occurrence of micronucleoli in the buccal mucosa increased to 0.38 +/- 0.07% vs. 0.19 +/- 0.06% prior to PC administration (p less than 0.01). Unscheduled DNA synthesis in peripheral lymphocytes decreased to T/C = 2.06 +/- 0.54 (p less than 0.01) compared to the value of T/C = 3.16 +/- 0.84 before PC administration. The lipid peroxidation level in the plasma was unchanged. The ascorbic acid administered simultaneously with the PC had no effect on the changes observed. The authors recommend a further follow-up of other side PC effects.