The solubilization of cattle achilles tendon with actinidin was investigated under neutral and acidic conditions. 1.43 to 1.92 and 0.97 to 3.19% of collagen were solubilized by treating the cattle achilles tendon with actinidin at 20°C at pH 6.0 and 3.3, respectively. Furthermore, SDS-polyacrylamide gel electrophoresis of reaction mixtures with actinidin demonstrated that actinidin degraded the tendon into collagen subunit chain, β- and α-chain and peptide fragments of various sizes at 20°C at pH 6.0 and 3.3. These results indicated that actinidin could solubilize the insoluble collagen in unheated cattle achilles tendon at 20°C at pH 6.0 and 3.3 and that a large proportion of the resulting peptide fragments by actinidin seemed to be actinidin digests against elastin with a small contribution of hydroxyproline.