Five heretofore undescribed analogues of methotrexate (MTX) and aminopterin (AMT) were synthesized and tested as dihydrofolate reductase (DHFR) inhibitors and tumor cell growth inhibitors. The meta isomer of AMT was obtained from 2,4-diamino-6-(bromomethyl)pteridine and m-(aminobenzoyl)-L-glutamic acid, while the ortho isomer was obtained via the same route by using alpha-methyl gamma-tert-butyl o-(aminobenzoyl)-L-glutamate instead of the free acid. Analogues of MTX and AMT containing a double bond in the side chain were prepared from dimethyl D,L-2-amino-4-hexenedioate and 4-amino-4-deoxy-N10-methylpteroic acid and 4-amino-4-deoxy-N10-formylpteroic acid, respectively. Finally, a positional isomer of MTX with the CH2CH2COOH moiety moved from the alpha-carbon to the adjacent carboxamide nitrogen was synthesized from 3-[N-(carboxymethyl)amino]propanoic acid diethyl ester and 4-amino-4-deoxy-N10-methylpteroic acid. The positional isomers of AMT were weak DHFR inhibitors and showed very little growth-inhibitory activity against L1210 murine leukemia cells or the MTX-resistant L1210/R81 mutant line in culture. The MTX and AMT analogues with the CH2CH2COOH moiety replaced by a CH2CH = CHCOOH side chain showed anti-DHFR activity similar to that of the previously described saturated compound N-(4-amino-4-deoxy-N10-methylpteroyl)-L-2-aminoadipic acid, but were less potent than the parent drugs. The MTX analogue with the CH2CH2COOH side chain displaced from C to N was weakly bound to DHFR, confirming the importance of an intact CONH moiety, and showed greatly diminished cell growth inhibitory potency relative to MTX. None of the compounds was a substrate for folylpolyglutamate synthetase (FPGS) from mouse liver. Furthermore, inhibition of folic acid polyglutamylation in vitro at equimolar 500 microM concentrations of drug and substrate was negligible. The structural changes embodied in these five novel compounds are therefore too great for binding to the FPGS active site.