Thalidomide was found to prolong the survival of skin allografts in some strains of mice when administered daily i.p. as a suspension in 0.5 % earboxymethyl cellulose (in normal saline.) The doses employed were 12.5 and 25 mg/mouse/day. Both donors and recipients were treated, commencing 10 days before grafting. No toxicity was noted at these dose levels.