The inhibitory activities of 60 4,6-diamino-1,2-dihydro-2,2-dimethyl-1- (X-phenyl)-s-triazines versus purified, recombinant Pneumocystis carinii (Pc) dihydrofolate reductase (DHFR) have been determined at pH 7.0. Utilization of these Kiapp values has led to the formulation of appropriate quantitative structure-activity relationships (QSAR's) for both meta- and parasubstituted derivatives. The QSAR's from Pc are compared with other triazine QSAR's derived versus chicken, murine tumor, Escherichia coli, and particularly human DHFR. Selectivity indices indicate that hydrophobic triazines are particularly effective versus Pc DHFR; they have lower Ki values for Pc DHFR than for human DHFR.