Propagation in human cells of a filterable agent from the ST feline sarcoma

Nineteen lines of human fibroblasts were inoculated with a filtrate prepared from the ST feline sarcoma. Seven lines showed morphological alteration and released focus-forming activity for feline cells, 2 lines showed morphological alteration but did not release focus-forming activity, and 11 lines showed no morphological alteration and released no focus-forming agent. Morphologically altered cells appeared enlarged, hyper-refractile, and intensely stained by hematoxylin. They neither assumed a crisscross pattern nor piled up to form a visible focus. Time-lapse cinegraph showed that the morphologically altered cells did not divide and were motile. The fluid from two human fibroblast cultures, inoculated 4 and 14 weeks previously with the ST sarcoma filtrate, induced fibrosarcoma in newborn kittens.