The environmental Mg2+ used in preparation of Bacillus subtilis membranes was found to influence the responses of the associated ATPase to cetyltrimethylammonium bromide (CTAB). Membranes prepared using fluids containing higher Mg2+ levels exhibited lower control activity than was seen with low Mg2+ membranes. Increased environmental Mg2+ resulted in higher stimulations with lower doses of the agent. ATPase of all three membrane types was stimulated in two concentration ranges, but in the doses tested, CTAB inhibited the ATPase of only those membranes obtained using fluids containing high Mg2+ for every stage of the isolation. Sonication of membranes for 25 s at half maximum output yielded three fractions, consisting of a soluble form which was sensitive to CTAB stimulation at 25 microg/ml of assay mixture; small, 95-110 nm, vesicles, which were resistant to CTAB at 25, 75, and 150 microg/ml, and large vesicles, similar to untreated membranes both in morphology and responses to detergent. Combinations of detergent and protein (beta-lysin or arginine-rich histone) produced activity appearing to be additive when the protein level was present in a high concentration and the detergent was present at levels corresponding to the minimum influence. Mixtures of a maximally stimulating dose (75 or 100 microg/ ml) of detergent and a small amount of protein produced activities that were at least 92% or more of the expected sums of individual stimulations. Interference occurred with the following mixtures: high amounts of detergent and protein; high protein and 10 or 15 microg/ml CTAB; and beta-lysin and arginine-rich histone, both at high levels. These data are consistent with a hypothesis that the two peaks in CTAB stimulation reflect two adjacent ATPase sites, one of which is also susceptible to stimulation by cationic protein.