Steady state and preliminary pre steady state studies of the mouse DHFR indicate that the wild-type enzyme used for our mutagenic studies follows a significantly different in vitro kinetic pathway than previously reported. In particular, turnover does not appear to be governed by H4F release from the E.NADPH complex. The discrepancies in catalysis and binding behavior of the mouse DHFRs may be due to the isomeric nature of the DHFRs studied. The enhanced ability of the two mutations at position 31 to confer resistance to MTX, as expected, decreased the affinity of the enzyme for the inhibitor. A correlation between the increased size of the side chain at position 31 and decreased inhibitor affinity was observed. This findings is consistent with previous mutagenesis studies of mouse DHFR but is at odds with conclusions drawn from an analysis of the role of the position in inhibitor binding to human DHFR. It is generally agreed that a highly efficient enzyme is desired for most cellular metabolic functions; however, because substitution of position 31 with tryptophan impairs catalytic efficiency, it appears that there exists a high physiological tolerance for significantly impaired DHFR. Indeed, mice who have received transplants of bone marrow expressing the Trp-31 mutant or the severely impaired Arg-22 mutant are capable of surviving lethal doses of MTX. Nevertheless, the consequences in vivo of a reduction in the observed in vitro catalytic effectiveness of DHFR remain to be determined. Additional mutagenic studies attempting to select catalytically silent mutations that reduce inhibitor binding may further enhance the therapeutic potential of drug-resistant DHFR genes for improved folate antagonist mediated antitumor activity.