Determination of load-extension curves of rat-tail tendon fibres showed that when extension was restricted to 2% the slope of the linear portion of the curves was unchanged in successive tests. The change of slope resulting from enzymic treatment was used to assess the damaging effect of hyaluronidase and trypsin. Pure hyaluronidase had no significant effect indicating that chondroitin sulphate A and C and hyaluronic acid are not important in stabilsing the fibres according to this criterion. Other preparations of hyaluronidase did damage the fibres and this is attributed to the presence of enzymic impurities which attack the interfibrillar matrix rather than the collagen of the fibres. Evidence is presented that it is the non-collagenous protein of the matrix that is attacked and which is important in stabilising the native fibres.