A spontaneously transformed hamster cell line, designated Clone B (Cl B), has been activated by halogenated pyrimidines to produce oncornavirus-like particles. The optimal conditions for virus induction were studied by the analysis of culture fluids for particles with polymerase activity. Maximal enzymatic activity was induced when exponentially growing Cl B cells were treated with 20 to 30 microgram/ml 5-iodo-2'-deoxyrudine for a 24-h period. Under these conditions, the kinetics of particle release reached a maximal level between 48 and 72 h following removal of the drug, and then decreased rapidly. The induced particles exhibited features characteristic of oncogenic RNA viruses: actinomycin D was found to inhibit their induction; the particles band in sucrose gradients at a density of 1.15 g/ml; nonionic detergent treatment releases the virion core component exhibiting a buoyant density of 1.21 g/ml in sucrose gradients; the viral particles contain high molecular weight RNA species with sedimentation coefficients of 65 S and 35 S; and the polymerase associated with Cl B particles efficiently transcribes synthetic templates preferred by the reverse transcriptases of known RNA tumor viruses, but demonstrates a very low endogenous activity.