The purpose of the present investigation is to report some histochemical and cytospectrophotometric observations providng more objective evidence for the specific activity of the histochemical method for dihydrofolate reductase activity (5.6.7.8-tetrahydrofolate: E.C. 1.5.1.3). Kinetic factors of the enzymic reaction, such as fixation, pH, coenzymes, substrate concentration, Michaelis constant, temperature, time of incubation, aerobiosis, and anaerobiosis, activators and inhibitors, were investigated.