Injuries of the equine superficial digital flexor tendon are common in racing horses. Knowledge of the tendon matrix composition is crucial to understand physiological and pathological processes in the tendon. The aim of this study was to analyze TSP-4 expressed in equine tendon. Equine tendons were extracted with 10 mM EDTA-containing buffer and TSP-4 purified with ion-exchange chromatography followed by heparin affinity chromatography. The purified TSP-4 was analyzed by one- and two-dimensional SDS-PAGE, immunoblotting, and MALDI-TOF mass spectrometry. Purified TSP-4 gave bands reacting with a TSP-4 specific antiserum, but also with an antiserum to COMP, when submitted to SDS-PAGE under nonreducing conditions. Two-dimensional SDS-PAGE (nonreducing followed by reducing conditions) and immunoprecipitation as well as MALDI-TOF mass spectrometry analysis showed that TSP-4 and COMP are both present in equine tendon and cannot be separated under nonreducing conditions despite significant differences in subunit size. This suggests that they are connected via disulfide bridges into heterooligomers.