Materials andMethods.-Collection of specimens andvirus isolation attempts in tissue culture: Specimens were obtained from employees of the National Institutes of Health with acute upper respiratory illness on or before the fourth day of illness. 12 Two-tenths ml of freshly collected nasopharyngeal wash fluid was inoculated into each of two tubes of the following tissue cultures: HEp-2, primary human embryonic kidney, rhesus monkey kidney, and human diploid cell strains (HDCS) WI-26, WI-38, and AT-39.13 The remaining fluid was stored at-60C. Tissue cultures were obtained from commercial sources, maintained as previously described,'and were incubated at 330C on roller drums. The tubes were observed for cytopathic effect twice weekly. Monkey kidney cultures were tested for hemadsorption at 5-to 7-day intervals, and a single blind passage of most human embryonic kidney cultures was made at 21 days. Only those nasopharyn-geal washings in which virus could not be detected by these methods were examined by organ culture techniques. Acute phase sera were drawn at the time the washings were collected and convalescent phase sera approximately 3 weeks thereafter. Organ cultures: Human embryonic tracheal organ cultures were prepared and maintained by a modification of the method of Hoorn and Tyrrell. 6, 7 Tracheas were obtained from fetuses spon-taneously aborted at 5-9 months' gestational age. The tissue was excised en bloc by sterile tech-nique and immediately stored in cold Hanks' balanced salt solution with 10% fetal calf serum,