The rare strains of staphylococci which lack either free coagulase or bound ceagulase (the clumping factor responsible for agglutination and compact colony formation in fibrinogen) , but not both these properties, have been considered to be od doubtful pathogenicity( 1). However, Johanovsky ( 2 ) suggested that the clumping property rather than free colagulase may allow survival ol f staphylococci within phagocytes. Hunt and Moses (3) reported that only one of 2 serologically distinct variants of the Smith strain of Staphylococcus uu~t'us was virulent for mice although both were coagulase positive. The mouse virulent variant formed diffuse colonies iin plasma soft agar, whereas the cololnies of the avirulent variant were compact in this mediurn. Alami and Kelly( 4) concluded that the coimpacting of staphylococcal colonies in fibrinogen-containing soft agar is an expression of the clumping factor and need not be due exclusively to antibody action. The present study was initiated to compare free coagulase anid bound coagulase as virulence factors and to determine whether route of injection affected the results in mice.Materials and nzefhods. Twelve strains representing typical coagulase positive and cmgulase negative staphylococci as well as variants which lack either free or bound coagulase (hereafter referred to as coagulase and clumping factor, respectively) or both these properties were selected for study. The compact colony (S) and diffuse colony (Sv) forming variants of the Smith strain were kindly supplied by Dr. G. A. Hunt. Others were isolated locally from hospital personnel and out-patients or were variants of these strains. Coagulase and clumping factor activities were determined by the soft agar technic(4) as well as by conventional methods.