IT has been recently shown that various strains of Streptococcus pyoqenes group A possess a cell-bound haemolysin (CBH) which can be released from the streptococcal cells into the surrounding medium by some surface active materials (Tween 40, Tween 80, Triton) by crystalline albumin, but not by sonlic energy (Ginsburg and Grossowicz, 1957 ; Ginsburg, 1958; Ginsburg and Grossowicz, 1959). This haemolytic factor was designated as streptolysin " D " (SLD) D signifying detergent. SLD is distinguished from streptolysin 0 (SLO) streptolysin S (SLS) (Todd, 1938) and from the intracellular haemolysin (IH) described by Schwab (Schwab, 1956) on the bases of different substrate requirements for formation, sensitivity to U.N. irradiation and to sonic energy (Ginsburg, 1958; Ginsburg and Gros- sowicz, unpublished). Besides haemolysing RBC of various animal species both cell-free and cell- bounid SLD, are also capable of injuring and killing various mamimalian cells in vitro (Ehrlich ascites tumour cells, fibroblasts, amnion cells, leucocytes) (Gins- buirg, 1958 ; Ginsburg and Grossowicz, 1959). The purpose of the present study is to show that Ehrlich ascites tuilour cells damaged by different streptococcal haemolysins may be disintegrated bv various proteolytic enzymes which by themselves are not lethal