The physical and biologic characteristics of a new synthetic absorbable monofilament suture, glycolide trimethylene carbonate (GTMC) are presented. The suture was formulated to combine predictable in vivo performance of synthetic absorbable sutures with the handling characteristics of a monofilament suture. Three in vivo studies were described: strength, gross and microscopic absorption and reaction, and radiolabelled decay. The studies carried out in rats showed cumulative strength retention of sizes 0, 00, 4-0 and 5-0 of 81 per cent at 14 days, 59 per cent at 28 days and 30 per cent at 42 days. Strength retention was consistent throughout the size spectrum. Absorption of sizes 00 and 4-0 were studied in subcutaneous implantations in rabbits. Histologic assessment of absorption obtained from serial sections at intervals of three to nine months showed that, in both sizes, complete absorption occurred between six and seven months. At six months, 83 per cent of size 00 was absorbed and size 4-0 was 93 per cent absorbed. At seven months, no implanted material was discernible histologically. Untoward tissue reactions, such as acute inflammatory cells, abscesses or tissue necrosis, were not observed. There were no signs of cellular mobilization of any kind observed remote from the implant. Absorption of GTMC sutures was achieved through the action of mononuclear and multinuclear macrophages which were confined to the implant and sequestered by a fibrous connective tissue capsule. When implant absorption was complete, resorbtion of the macrophage component was observed which was replaced by a narrow cord of fibrous tissue and collagen. The results of studies of radiolabelled sutures carried out in the subcutaneous tissues of rats revealed urine and expired CO2 to be the major excretary routes of the metabolites. By 22 to 24 weeks, 0.1 to 0.7 per cent of the total implanted radioactivity remained at the suture sites. Tissue deposition and excretion of radioactivity suggests similar metabolism of the sutures in both species. We conclude that GTMC sutures maintain good strength with little or no absorption during the critical wound healing period, absorbs completely from tissues in six to seven months with minimal tissue reaction and, therefore, provides an absorbable, flexible, monofilament material with extended support that is strong and effective.