Membranes of corn (Zea mays, cv Trojan 929) coleoptiles were fractionated by sucrose density gradient centrifugation and the locations of organelles were determined using marker enzymes and electron microscopy. Latent IDPase (or UDPase) was selected as the Golgi marker and UDPG-sterol glucosyl transferase was selected as the plasma membrane (PM) marker, because they were clearly separable from markers for the other organelles. Golgi-rich and PM-rich fractions were studied in relation to their ATPase activities. The pH optimum of the KCl, Mg(2+)-ATPase of the PM-rich fraction from a step gradient was 6.0 to 6.5, while the Golgi-rich fraction had peaks at pH 6.0 to 6.5 and pH 7.5. It is hypothesized that the peak at pH 6.0 to 6.5 for the Golgi-rich fraction is due to PM-contamination, while the peak at pH 7.5 represents the activity of a Golgi ATPase. To reduce PM contamination, Golgi-rich fractions obtained from step or rate-zonal gradients were recentrifuged isopycnically on linear sucrose gradients. The distribution of KCl, Mg(2+)-ATPase activity was measured at pH 6.5 and 7.5. The pH 6.5 ATPase was coincident with UDPG-sterol glucosyl transferase, a PM marker, while the pH 7.5 ATPase overlapped with latent UDPase, a Golgi marker. These results provide strong evidence for a KCl, Mg(2+)-ATPase, active at pH 7.5, associated with the Golgi membranes of corn coleoptiles.