Three to six mg of the millimolar Ca2+-requiring proteinase (m-calpain) were obtained from 1 kg bovine cardiac muscle (fresh wt) and some enzymatic properties of this proteinase were determined. Activity of bovine cardiac m-calpain decreases as ionic strength increases from 75 to 1000 mM. Maximal activation of m-calpain by Ca2+, La3+, Ba2+, and Mn2+ occurs at 2 to 3 mM concentrations of each of these divalent cations, but La3+ activation is only 20 to 25% and Ba2+ and Mn2+ activation only 6 to 10% as great as Ca2+ activation. Maximum Sr2+ activation occurs at 20 mM Sr2+ and is 90 to 95% of maximum Ca2+ activation. Mg2+, Zn2+, Cr2+, and Cd2+ do not activate m-calpain when added alone; Mg2+ does not affect, but Zn2+ inhibits, Ca2+-stimulated activity. The nonionic detergents, Triton X-100 and Brij 35, activate m-calpain 1.6- to 2.0-fold but do not change its Ca2+ requirement. Sodium dodecyl sulfate and urea inhibit m-calpain completely at 0.045% and 2.0 M, respectively. Because they bind Ca2+ needed for activation, ATP, ADP, and ITP inhibit m-calpain. The trypsin inhibitors, phenylmethylsulfonyl fluoride, ovomucoid trypsin inhibitor, ovoinhibitor, aprotinin, alpha 1-antiproteinase inhibitor, soybean trypsin inhibitor, and lima bean trypsin inhibitor do not affect m-calpain activity; m-calpain does not release measureable quantities of acid-soluble peptides from a rabbit skeletal sarcoplasmic protein fraction but does degrade rabbit skeletal myofibrils and casein.