THE main obstacle to the characterization of reticulin has always been its close association with collagen. As a preliminary to an attempt to establish the nature of reticulin fibres, a histological survey of all likely tissues was carried out, and it was found that the mid-cortical connective tissues of the kidneys of children free from renal disease consist almost entirely of reticulin. The only collagen present in this situation occurs in the immediate neighbourhood of the larger blood vessels.