The enzymatic activity of phosphoglycerate mutase (Pgm) from three gram-positive endospore-forming bacteria (Bacillus subtilis, Clostridium perfringens, and Sporosarcina ureae) requires Mn2+ and is very sensitive to pH; at low concentrations of Mn2+, a pH change from 8 to 6 resulted in greater than 30- to 200-fold decreases in the activity of these Pgms. However, Pgm deactivation at pH 6 was reversed by shifting the enzyme to pH 7 or 8. Free Mn2+ was not directly involved in Pgm catalysis, although enzyme-bound Mn2+ may be involved. The rate of catalysis by Mn(2+)-containing Pgm was also slightly pH dependent, although the Km for 3-phosphoglyceric acid appeared to be the same at pH 6, 7, and 8. These findings suggest that Mn2+ binds to catalytically inactive Pgm and converts it to a catalytically competent form, and further, that pH influences the efficiency with which the enzyme binds Mn2+. The extreme pH sensitivity of the Mn(2+)-dependent Pgms supports a model in which this enzyme is inhibited during sporulation by acidification of the forespore, thus allowing accumulation of the spore's large depot of 3-phosphoglyceric acid. The activity of Pgm from two closely related gram-positive bacteria that do not form spores (Planococcus citreus and Staphylococcus saprophyticus) also requires Mn2+ and is pH sensitive. In contrast, the Pgm activities from two more distantly related non-endospore-forming gram-positive bacteria (Micrococcus luteus and Streptomyces coelicolor) are neither dependent on metal ions nor particularly sensitive to pH.