This paper describes the study of a highly purified pyrimidine nucleoside monophosphate kinase from rat bone marrow cells. Short-term storage (24 h at 4 degrees C) of the purified enzyme in the absence of dithiothreitol, a sulfhydryl reducing agent, led to considerable losses of enzyme activity. Most of the lost activity could be regained, however, by incubating the enzyme with 50 mM dithiothreitol. Enzyme stabilization by dithiothreitol and reactivation by dithiothreitol were enhanced in the presence of phosphate buffer. Severe enzyme inhibition was produced by micromolar concentrations of sulfhydryl group reagents. Chromatographic, electrofocusing, and sucrose gradient centrifugation experiments revealed that the enzyme has a molecular weight of about 26,000, an isoelectric point of 4.7, and a sedimentation coefficient of 2.5. These experiments were also carried out with enzyme preparations which had been almost completely inactivated by means of dialysis to remove dithiothreitol. Enzyme preparations of this type displayed at least one additional enzyme form. This form(s) was inactive but capable of being partially reactivated by dithiothreitol. The inactive form(s) exhibited the same apparent molecular weight as the native enzyme but possessed a higher isoelectric point (5.7). A working hypothesis was presented which states (1) that inactive enzyme forms arise because of disulfide bond formation, (2) that enzyme sulfhydryl groups are less susceptible to oxidation in the presence of phosphate buffer, and (3) that enzyme reactivation by dithiothreitol results from the regeneration of critical enzyme sulfhydryls.