Abstract: O⁶-Alkylguanine- DNA- alkyltransferase (O⁶- AGT ) is a very important DNA repair protein known to carry out the transfer of alkyl groups from the O⁶ position of guanine in alkylated DNA to a cysteine acceptor site contained within its own protein sequence. In this work, the activity of O⁶- AGT in different cell lines and the relationship between the depletion of the enzyme and the frequency of micronuclei induced by cisplatin (DDP), Ning Xin platin (camphoramine chloroacetic platinum, CCP) or carboplatin (JM-8 ) in KB and CHL cells were studied. Experiments indicate that KB cells showed higher O⁶- AGT activity (> 400 dpm/300μg protein extracts ) which belonged to Mer⁺ cells, but CHL, HL-60 and L1210 cells showed very low O⁶- AGT activity (<50 dpm/300μg protein extracts) which can be considered to be Mer^- cells. Cytotoxicity studies indicated that no mer^- selection was observed in these platinum complexes for KB, HL-60, CHL and L1210 cells. However, a good relationship between the depletion of O⁶- AGT and the frequency of micronuclei induced by the platinum complexes was obtained. CCP caused the highest depletion of the enzyme and exhibited highest potency in damaging chromosome.