Materials and Methods.-Strains of virus: One preparation of the" Schmidt-Ruppin" strain of RSV was obtained from Dr. W. Nichols of the South Jersey Medical Foundation, who had in turn received it from Professor Ahlstrom of Lund, Sweden. It was received in the form of a frozen fragment of tumor excised from a chicken of a leukosis-free flock which had been injected intramuscularly with the virus. After thawing, the tumor tissue was finely minced, suspended in phosphate-buffered saline at five times the volume of its net weight, and the suspension rapidly frozen and thawed three times. The suspension was then centrifuged for 20 min at 2000 rpm, and the supernatantusedas inoculum forthetissue cultures. Thispoolisreferredtoas SR-1. Itstiter in chick fibroblasts was 106 focus-forming units (FFU) per ml; 0.3 ml of undiluted material was used as inoculum for one milk dilution bottle of tissue culture cells.