Materials and Methods.-The macrophages were obtained byexplanting frag-ments of liver from newborn (1-3 days old) mice into roller tubes either directly onto the glass or onto a reconstituted collagen substrate. 3'4 The collagen was pre-pared according to the method of Ehrmann and Gey by extraction of 0.1 per cent acetic acid, dialysis against distilled water, and reconstitution to an agar-like slant with ammonium hydroxide vapors. The supernatant medium, except in otherwise specified cases; consisted of 60 per cent Gey's balanced salt solution, 10 per cent chick embryo extract (50 per cent), and 30 per cent horse serum (obtained from Microbiological Associates), with 0.004 per cent phenol red, 100 units of penicillin, and 10 micrograms of streptomycin. In some experiments, as designated in the text, chicken serum or a combination of chicken serum and horse serum was used. The chicken serum was obtained from White Leghorns kept for routine bleeding in our laboratory. The cultures were incubated in a roller drum and maintained in this manner at 370C throughout the experiments. The cultures were inoculated with the virus three to four days following explantation. The medium was renewed every two to three days until the end of the particular experiment. Supernatant fluids were frozen at-40'C and reserved for titration in mice. The cells were observed directly in the roller tube and their