Latent ATPase, located on the inner surface of protoplast ghosts of Mycobacterium phlei, was unmasked either by trypsin or an impermeable form of trypsin, ethylene maleic anhydride-trypsin. Density gradient experiments showed that the ghost preparations remained intact following trypsin treatment. Evidence was obtained that 125I-trypsin failed to penetrate the ghost membranes. Thus, attempts were made to determine whether the ATPase molecule in the ghost membranes is accessible from the outer surface. Treatment of protoplast ghosts and trypsin-treated ghosts with 125I by the lactoperoxidase method resulted in the labeling of ATPase only in the trypsin-treated ghost preparations. The antibody to latent ATPase inhibited ATPase activity in trypsin-treated ghosts. The changes in the fluorescence polarization of diphenyl hexatriene indicated that trypsin treatment of the ghost membranes resulted in an increase in membrane fluidity. These studies suggest that the latent ATPase moiety has undergone translocation to the outer surface or it became accessible to trypsin digestion from the outer surface of the membranes as a result of removal of some proteins covering ATPase molecule in the membranes.