The production of a clotting factor (peptocoagulase) by bovine clinical isolates of Peptococcus indolicus and its nature were investigated. Extracellular peptocoagulase was demonstrated in culture filtrates of 93% and cell associated with washed cell suspensions of 100% of the 75 isolates tested. Both citrated and heparinized plasma were clotted. Crude peptocoagulase was nondialyzable, precipitated with (NH4)2SO4 at 40% saturation, somewhat resistant to heating at both neutral and acid pH, and chloroform insoluble. Culture filtrate did not contain proteolytic activity with albumin and casein, as substrates and no esterase activity was detected with tosylarginine and benzoylarginine methyl esters as substrates. The clotting reaction required peptocoagulase, prothrombin, and fibrinogen. The activation of prothrombin appeared to involve a stoichiometric reaction with peptocoagulase, possibly by formation of a stable complex.