The antitumor agent cis-diamminedichloroplatinum(II) (cisplatin) introduces cytotoxic DNA damage predominantly in the form of intrastrand crosslinks between adjacent purines. Binding assays using a series of duplex oligonucleotides containing a single 1,2 diguanyl intrastrand crosslink indicate that human cell extracts contain factors that preferentially recognise this type of damage when the complementary strand contains T opposite the 3', and C opposite the 5'guanine in the crosslink. Under the conditions of the band-shift assay used, little binding is observed if the positions of the T and C are reversed in the complementary strand. Similarly, duplexes containing CC or TT opposite the crosslink are recognised relatively poorly. The binding activity is absent from extracts of the colorectal carcinoma cell lines LoVo and DLD-1 in which the hMutSalpha mismatch recognition complex is inactivated by mutation. Extensively purified human hMutSalpha exhibits the same substrate preference and binds to the mismatched platinated DNA at least as well as to an identical unplatinated duplex containing a single G.T mismatch. It is likely, therefore, that human mismatch repair may be triggered by 1,2 diguanyl intrastrand crosslinks that have undergone replicative bypass.