Monoclonal antibodies (mAbs) against antipterin immunoglobulin and dihydropteridine reductase (DHPR) and also polyclonal antibodies against human dihydrofolate reductase (DHFR) were obtained. The anti-idiotypic mAbs and anti-DHPR mAbs bind specifically to human DHFR, Escherichia coli DHFR, soybean seedling DHFR, and human DHPR in solidphase immunoassays. Further, the mAbs bind to the native but not to the denatured forms of DHFRs. The monoclonal antibodies also inhibit the enzymatic activity of human DHFR but not that of human DHPR. Competitive solid-phase immunoassays show stoichiometric inhibition by methotrexate and partial inhibition by NADPH of mAb binding to human DHFR. Cyanogen bromide fragments derived from human DHFR (residues 15-52 and 53-111), containing several active site residues, bind partially to some of the monoclonal anti-bodies. Accordingly, polyclonal antibodies to peptide 53-111 of human DHFR cross-react to some extent with human DHPR. Data from competitive immunoassays in which the binding of the various mAbs was tested singly and in combination with other mAbs suggest that these antibodies bind to a common region on human DHFR. The results also indicate that the mAbs display some heterogeneity with respect to specific epitopes. These data suggest that despite the absence of significant amino acid sequence homologies among the various DHFRs and DHPR, they have a fundamentally similar topography at the site of binding of the pterin moiety that is recognized by the anti-idiotypic mAbs generated by pterin. In the relatively simple structure of the pterin ring system there are different substituent groups at positions C4 and C6 in methotrexate, 7,8-dihydrofolate, and 7,8-dihydrobiopterin, suggesting that these antibodies are specific for regions on various proteins that interact with the remainder of the pterin moiety. These mAbs and similar mAbs specified by substituent groups on pterin may thus be used as specific probes or inhibitors of various folatedependent enzymes and transport proteins. They should also provide insights into some of the general features of antibody recognition of protein antigens.