Treatment with 1 mM hydroxyurea (HU) for 12 h induced sister-chromatid exchange (SCE) in CHO-K1 cells. The induced SCE frequency was always higher in cells grown in Ham's F12 medium than in those grown in RPMI 1640 medium. It was shown that hypoxanthine (Hyp), a component of Ham's F12, was to a great extent responsible for producing a higher level of HU-induced SCEs: HU-induced SCEs were synergistically enhanced when Hyp was added to RPMI 1640 medium supplemented with dialyzed fetal bovine serum at a concentration of 30 microM, which is the concentration in Ham's F12 medium. The radioactivity of [14C]Hyp was readily incorporated into DNA in either the presence or the absence of HU. The greater part was in the forms of dGMP and dAMP. It was not clear whether Hyp was incorporated in the form of dIMP or not. Deoxyguanosine (dGuo), but not deoxyadenosine (dAdo) reversed both the incorporation of radioactivity into DNA and the SCE-enhancing effect of Hyp. Our results indicate that incorporation of Hyp into the dNTP pools and into the DNA, together with perturbation of dGuo metabolism under abnormal conditions during and after HU treatment, is involved in the enhancement by Hyp of HU-induced SCEs.