In this study, the anticlastogenic effects of ascorbic acid and the protective effect of folinic acid against the formation of chromosomal aberrations in humans by pyrimethamine were investigated. Pyrimethamine is a folic acid antagonist used for the treatment of malaria and toxoplasmosis. In this study, 18 different healthy people, who do not drink alcohol and are non-smokers, were chosen as an experimental group; 0.025 mg/ml pyrimethamine was given to the lymphocyte culture, which had been prepared with the peripheral blood taken from this group. After that each of the following doses were given to the same culture: 20, 40, and 80 mM of ascorbic acid and 25, 50, and 100 mM of folinic acid. The results of the cytogenetic evaluation showed that the aberrations due to pyrimethamine in the chromosomes were reduced by ascorbic acid and folinic acid significantly, depending on the given dose. (C) 2002 Wiley-Liss, Inc.