Allison A. Stewart, DVM, MS; Jennifer G. Barrett, DVM, PhD; Christopher R. Byron, DVM, MS; Angela C. Yates, DVM, MS; Sushmitha S. Durgam, BVSc; Richard B. Evans, PhD; Matthew C. Stewart, BVSc, PhD American journal of Veterinary Research June 2009 70 6 750-757 Objective—To compare viability and biosynthetic capacities of cells isolated from equine tendon, muscle, and bone marrow grown on autogenous tendon matrix. Sample Population—Cells from 4 young adult horses. Procedures—Cells were isolated, expanded, and cultured on autogenous cell-free tendon matrix for 7 days. Samples were analyzed for cell viability, proteoglycan synthesis, collagen synthesis, and mRNA expression of collagen type I, collagen type III, and cartilage oligomeric matrix protein (COMP). Results—Tendon- and muscle-derived cells required less time to reach confluence (approx 2 weeks) than did bone marrow–derived cells (approx 3 to 4 weeks); there were fewer bone marrow–derived cells at confluence than the other 2 cell types. More tendon- and muscle-derived cells were attached to matrices after 7 days than were bone marrow–derived cells. Collagen and proteoglycan synthesis by tendon- and muscle-derived cells was significantly greater than synthesis by bone marrow–derived cells. On a per-cell basis, tendon-derived cells had more collagen synthesis, although this was not significant. Collagen type I mRNA expression was similar among groups. Tendon-derived cells expressed the highest amounts of collagen type III and COMP mRNAs, although the difference for COMP was not significant. Conclusions and Clinical Relevance—Tendon- and muscle-derived cells yielded greater cell culture numbers in shorter time and, on a per-cell basis, had comparable biosynthetic assays to bone marrow–derived cells. More in vitro experiments with higher numbers may determine whether tendon-derived cells are a useful resource for tendon healing.