Retrovirus-like particles produced by human embryonal cells and cell lines derived from human malignancies.

Very small amounts of retrovirus-like particles were obtained from tissue culture fluids of various human cell lines. The particles were found in almost all cultures of rapidly growing human cells (embryo fibroblasts, various types of leukemias, melanoma, urinary bladder, lung and mammary carcinomas). Morphology and some biochemical characteristics of purified samples of these particles are presented. The particles resemble C-type mammalian retroviruses.