1. 1. We used trypsin digestion of two purified forms of the ATPase from Micrococcus lysodeikticus strain B combined with gel electrophoresis in regular and sodium dodecyl sulphate gels to study their structural differences and relationships. 2. 2. One of the forms, designated as form B AT (from strain B, active, stimulated by trypsin) was very resistant to trypsin. Controlled tryptic digestion led to stimulation of its ATPase activity and transformation into molecular species migrating faster in regular gels. 3. 3. The original subunit pattern of form B AT consisted of α (molecular weight 60 000), β (50 000), γ (32 000), ε (25 000) and components of relative mobility 1.0. It was modified by controlled tryptic cleavage to give a degradation product of α of molecular weight 52 000 comigrating with the β subunit and a destruction of the ε subunit. However, the β and γ subunits and the degradation product of α were very resistant to further trypsin digestion. 4. 4. A component β′ (molecular weight 40 000) which was absent in the original B AT form, appeared during its trypsin digestion. This result is discussed in relationship to the significance of a subunit of the same molecular weight found in the ATPase of a closely related strain of M. lysodeikticus (Andreu, J.M., Albendea, J.A. and Muñoz, E. (1973) Eur. J. Biochem. 37, 505–515). 5. 5. A form B I (from strain B, low activity form) which was non-stimulated by trypsin and may result from partial inactivation of form B AT was, however, very sensitive to trypsin. The transformation of B I into faster migrating components was therefore limited. The original subunit pattern consisted of α, β (both with molecular weight about 50 000), γ (32 000) and components of relative mobility 1.0. The sensitivity to trypsin of form B I was paralleled by that of the α, β and essentially γ subunits. It is suggested that a mechanism more complex than a trypsin-like endogenous proteolysis is involved in the transformation of form B AT to B I. 6. 6. Some of the transformations detected by gel electrophoresis leading to stimulation of ATPase activity in form B AT were paradoxically similar to those resulting in inactivation (form B I). This suggests that the analysis by gel electrophoresis