The molecular structure of quinespar, a quinazoline analogue of methotrexate and aminopterin, has been determined by X-ray crystallography. The molecule displays an extended conformation with the p-aminobenzoyl plane rotated 66 degrees from the plane of the quinazoline. The orientation of the quinazoline ring relative to the rest of the molecule is intermediate between the orientations of the comparable pteridine rings in folic acid and in DHFR-bound methotrexate. Evidence is presented to suggest that 2,4-diaminoquinazolines bind to DHFR in the same manner as do 2,4-diaminopteridines.