Silicone gels derived from commercially obtained implants induce plasmacytomas in 60-70% of highly susceptible BALB/cAn.DBA/2-Idh1-Pep3 congenic mice. In contrast, dimethylpolysiloxane (DMPS) silicone oils with viscosities of 5, 1000 and 12,500 cs fail to elicit these tumors. 1000 cs vinylmethylpolysiloxane is also inactive. Silicone gels, in contrast to the oils, induce a highly inflammatory silicone granuloma. Silicone gels contain chemical components not found in the oils. The chemical component responsible for inducing the permissive environment for plasmacytoma formation has not yet been identified. Silicone gels are well tolerated for long periods of time in mice without adverse effects other than plasmacytoma formation. The response to different gel preparations varies; some are associated with relatively rapid formation of plasmacytomas resembling that seen with pristane, while in others the plasmacytoma formation is extended nearly over a two year period.