We prove an abstract mean ergodic theorem and use it to show that if {A(n)} is a sequence of commuting m-dissipative (or normal) operators on a Banach space X, then the intersection of their null spaces is orthogonal to the linear span of their ranges. It is also proved that the inequality \\x + Ay\\ greater than or equal to \\x\\-2 root\\Ax \\y\\ (x, y is an element of D(A)) holds for any m-dissipative operator A. These results either generalize or improve the corresponding results of Shaw, Mattila, and Crabb and Sinclair, respectively.
