Methods.-Acid-soluble calf skin collagen was prepared as described previously5 and stored in the lyophilized state at-20'C. Solutions were prepared by dissolving 10 mg of collagen in10 ml of cold sterile phosphate buffer, pH 7.6, r/2= 0.4, by shaking overnight at 50C. The viscous solution was then dialyzed 24 hragainst 0.4 M NaCl and clarified by centrifugation at 100,000 g for one hr. Radioactive guinea pig skin collagen was prepared from actively growing 250 gm animals injected intraperitoneally with 100 1c of C14 glycine 6 hr prior to sacrifice. Collagen was extracted from the dermis with 0.14 M NaCl or 0.45 M NaCl. 6 The solubilized collagen was purified by repeated salt precipitation.'The specific activity of the batch used in these experiments was 20,000 cpm/mg of protein. Simpleculture cells were constructed by sealing (with poly-vinyl acetate) two pairs of concentric plastic rings onto standard microscope slide. Dimensions of the inner rings are 13 mm OD, 9 mm. ID, and 1.7 mm deep. The outer rings, provided to facilitate bathing the gels with medium are 16mm ID, 19 mm OD, and 3 mm deep. Cold collagen solution (125 Al of approximately 0.1% collagen in 0.4 M NaCl) was added to the central chamber toa level flat with the top of the ring and incubated in a moist atmosphere at 370C for at least three hr. At this temperature, collagen in neutral solution will precipitate as a mass of typical cross-striated fibrils in an opalescent gel. 5 These relatively rigid gels were bathed with warm sterile mediumcontaining 150 units eachtof penicillin and streptomycin per ml, then drained of free fluid. Thus, the saline of the gel was replaced by a physiologic medium. Larger cultures were prepared in 30 mm culture dishes on 2 ml of collagen gel. Rana catesbiana tadpoles, stage IX to XXI (Kollros scale for R. Pipiens) 8 were sterilized prior to useby keeping them in dishes containing 500,000 units of penicillin and 750mg streptomycin per liter for 12 to 18 hr. Small pieces of tissue 0.5 to 4 mm across werewashed in Tyrode solution and applied to the surface of the collagen gels without adding additional medium. The chambers covered with square glass slips were incubated at 270 or 370C in a moist atmosphere containing 5 per cent CO2. Cultures were observed on a side-illuminated black platform stage of a dissecting microscope. The lysed area showed as a black hole in a white opalescent disk. Areas of explant and lysis were measured with an ocular micrometer. Whole stained mounts of cultures were prepared by fixing in 10 per cent formalin, removing the rings, staining in anilineblue, and sealing in balsam with a cover slip. Radioactivity in solution fromdegraded collagen was determined after 50,000 g centrifugation at room temperature. The supernatant fluids were dried and counted in a low-background gas flow counter. Throughout the experiments, the pHof the culture media was checked after incubation and found in all cases to be between 7.2 and 8.2. Sterility of the cultures was frequently con-firmed by inoculating the medium in broth and agar plates. Bacterial contamination, when it infrequently occurred, was clearly visible as discrete colonies which in no case produced lysis of