Strain HF (type 1) produces large plaques with a single flat polykaryocyte, whereas strain KY (type 2) produces small ones with round cells. Four recombinants were obtained by cross of HF X KY which possessed the characters intermediate between the parents with respect to plaque size and cytopathogenic effect. Further crosses between these recombinants produced HF-type virus. The crosses between the one recombinant and fresh isolates or mutant viruses also produced HF-type virus at different ratios.