Using either DEAE-cellulose chromatography or pH 4--6 isoelectric focusing, we have separated mouse skeletal muscle neutral RNAase II-inhibitor complex into two fractions (designated alph and beta). para-Hydroxymercuriphenyl-sulfonate-induced dissociation/inactivation of the inhibitor yields free RNAase II enzyme fractions with differing pH profiles, CM-cellulose chromatographic behavior and reactivity with the RNAase II inhibitor of human placenta. However, the free RNAase fractions react equally with purified inhibitor from skeletal muscle and are not separable by pH 8--9.5 isoelectric focusing. These data suggest that mouse skeletal muscle has two heterogeneous forms of RNAase II. Additionally, heterologous RNAase II inhibitors may be used as investigational tools when probing neutral RNAase II heterogeneity.