Rat embryo fibroblasts were abortively infected with various stocks of herpes simplex virus type 1 strain ANG at 42 degrees. In uninfected controls all of the cells died during an 8-day incubation period at the elevated temperature, whereas varying numbers of cells in the infected cultures survived and formed colonies during subsequent incubation for 3--4 weeks at 37 degrees. All of the survivors appeared to be morphologically transformed. Two types of survivors, epithelial- and spindle-like cells, which occurred at a ratio of approximately 1:1 in all assays, could be distinguished. The observed survival rates increased from about 1 x 10-7 to 3 x 10-5, corresponding to increasing fractions (0--50%) of a defective genotype present in the infecting virus stocks. The individual survival rates do not depend exclusively on the quantity of defective virions. The existence of different subtypes of defective genomes as a further parameter is discussed.