This study measured the amount of immature collagen type 111 present in tendon rafts obtained from anterior cruciate ligament (ACL) reconstructions. These values werc compared with those obtained from control grafts typically used for reconstructionAchilles, patellar, and fascia lata -and also to the normal ACL. Andyses were performed using a commercially available radioimmunoassay (RIA). The RIA made use of a rabbit polyclonal antibody specific to the amino terminus of procollagen type 111. The specificity of the Ab was confirmed by a western blot. Fibril diameter of each of the above samples was measured by transmission electron microscopy (TEM). We thus were able to determine if there was a relationship between pN collagen 111 content and fibril diameter.The mean amount of pN collagen type 111 in the normal tendon control group was 0.8 f 0.3 nglpg total protein (range 0.0-2.5 ng/pg). There was significantly greater pN collagen 111 (16 i 3.7 ng/pg total protein) in the grafts containing an average fibril diameter <55 nm than in the normal tendons or ACL ( P < 0.05). Grafts with an average fibril diameter >55 nm had similar levels of pN collagen 111 (1.0 i 0.79 ng/pg) as the controls. There was also significantly less pN-collagen 111 within the functional grafts (5.3 f 1.9 ng/pg) as compared to failed grafts, (21.6 f 5.1 ng/pg, P < 0.05).These results indicate that incomplete processing of procollagen I11 may be responsible for some of the ultrastructural alterations seen in tendon grafts. Since ultrastructural organization is believed to influence mechanical properties of these tissues, p N collagen I11 levels may be a possible indicator of ligament or tendon weakness.