McKee,1 of the University of Iowa department of bacteriology, reports the production of a nontoxic variant of influenza A virus, with immunizing properties superior to those of routine toxic influenza A virus. The virus used in this study was originally isolated from the 1943 outbreak in Iowa City and had been passed repeatedly in chick embryos but had never been in mice. Two methods of subculture were adopted. By the first method, 0.5 cc. of freshly harvested undiluted allantoic fluid (hemagglutination titer 1:6,400) was inoculated into the allantoic sac of a 11 day old chick embryo. By the second method similar inoculation was made with a 1:10,000 dilution of the same fluid. All inoculums contained 500 units per cubic centimeter of pencillin and streptomycin.
After 24 to 48 hours of incubation at 37 C., the hemagglutination titer was the same (1:6,400) in both series. The two series, however, differed