Some plant extracts and products are known to affect mammalian cells, tissues and organisms as they contain a toxic substance or a metabolic stimulant. Our biochemical investigations revealed that some plant saponins can increase the cellular DNA repair activity and the general recombinase activity measured by in vitro assay (1). In the experiments described here, HeLa cells were cultured for several days with plant saponins or flavonoids and analyzed to measure i) recombination activity of the cell extract by induction of Tcr colonies from two mutant DNAs (mutants 1 and 2, which are both tetracycline sensitive) after transformation into E. coli recA-, and ii) repair synthesis of nuclear DNA followed by incorporation of 3H-thymidine. Saikosaponins a, bl, d, ginsenosides Rbl, Re, Rh and flavonoid baicalin caused a significant stimulation of intermolecular recombination. It is worth noting that none of the plant saponins and flavonoids had any inhibitory or toxic effect at concentrations less than 25 μg/ml in the culture media.