The induction of DNA repair was investigated in cultured Syrian hamster embryo (SHE) cells and Chinese hamster ovary (CHO) cells by cesium chloride equilibrium gradient sedimentation techniques following exposure to NiCl2, amorphous NiS, crystalline NiS and crystalline Ni3S2. Significant repair was induced in CHO cells by 1 microgram/ml of crystalline NiS following 24 h of treatment while 5 micrograms/ml caused more than twice the repair activity. In contrast amorphous NiS at 10 micrograms/ml for 24 h induced little repair in these cells. Similarly amorphous NiS did not induce repair at 5-10 micrograms/ml for 24 h in SHO cells while crystalline Ni3S2, and NiCl2 caused substantial induction of DNA repair synthesis at 10 micrograms/ml or 100 microM, respectively. These results demonstrate that nickel compounds which are potent transforming agents and induce damage to DNA also result in the induction of DNA repair. Repair synthesis was detected at concentrations of metal compounds which result in no detectable damage to DNA.