When infected with rubella virus, many human diploid fibroblast cell strains show mild to marked degrees of mitotic inhibition (PLOTKIN et al.: Amer.J. Epidem. 81: 71 [1965]); cell strains derived from infected fetuses divide more slowly than normal (RAWLS and MELNICK: J.exp.Med. 123: 795 [1966]); and infants congenitally infected with rubella show mitotic inhibition and growth retardation (NAEYE and BLANC : J.amer.med. Ass. 194: 1277 [1965]). In seeking an explanation for these phenomena a substance inhibiting mitosis was found in infected WI-38 cells. WI-38 cells that had gone into mitotic arrest two weeks after rubella virus infection were extracted by freeze-thawing. When freed of virus, extracts of infected cells induced mitotic inhibition of normal WI-38 cells and of a skin fibroblast cell strain. No effect was seen on continuous lines of human cells or on non-human cell lines. The inhibitory substance was trypsin-sensitive and heatlabile but was unaffected by rubella antibody, ether, ribonuclease, or deoxyribonuclease. It was not sedimentable or acid-stable and did not protect cells against the action of vesicular stomatitis virus. Concentrated rubella virus inactivated by ultra-violet irradiation did not itself produce growth inhibition. This substance offers a potential explanation for the mitotic inhibition associated with rubella virus infection in vitro and in vivo. (SPR)