A systematic comparison has been made of six suture materials used as subcuticular closure of abdominal incisions in dogs. The effect on wound healing was measured by mechanical, biochemical, and histologic methods. No difference was demonstrated in wound breading strength among wounds closed with different suture materials up to 28 days postoperatively. At 70 days, wounds sutured with nonabsorbable sutures were weaker than those closed with absorbable sutures, but this difference probably was due to a much higher incidence of infection in wounds closed with nonabsorbable sutures. It was our observation that monofilament sutures were superior to multifilament sutures with regard to the incidence of wound infection. By five days, the rate of collagen synthesis in wounds was increased over that of normal skin and remained elevated throughout the 120 day observation period. Suture material had no effect on collagen synthesis. The rate of noncollagenous protein synthesis in the wound was not altered throughout the entire observation period and did not differ from that measured in normal skin. Catgut, both plain and chromic, produced only a mild cellular reaction in dogs after 21 days as contrasted with the intense inflammatory reaction reported by others in rats and rabbits. We observed neither plain nor chromic catgut was absorbed rapidly in dogs; intact sutures were frequently observed at 120 days. Polyglactin, a synthetic absorbable suture, produced a moderate tissue reaction and uniformly disappeared between the twenty-eighth and seventieth days. Silk and Mersilene showed the highest rate of wound infection and the most intense and prolonged tissue reaction. Prolene, a monofilament suture, produced only a mild to moderate tissue reaction. These results taken in conjunction with those of other investigators suggest a marked species difference in the reaction to suture materials, particularly catgut, and suggest caution in transferring these observations to human beings.