Severe reactions can be caused when elastic impression materials are trapped beneath soft tissues. To develop a rank order of severity of responses, a series of these materials was injected beneath the oral mucosa of a baboon. Alginate and polysulfide rubbers containing lead peroxide produced severe reactions; hydrocolloid, polysulfide without lead, silicone, and polyether produced more mild reactions. The more toxic materials proved to be more radiopaque than the others. Tests for foreign body toxicity should be made a part of future specifications for these materials, and the use of toxic ingredients should be reduced as much as possible. These materials should be made opaque to X rays and of a different color than those of soft or hard tissues.