“…Subsequently White, Barlow, and Mozen (6), who, by alternative methods prepared urokinase of comparable specific activity to that of Lesuk et al, described preparation inhomogeneity and the presence of at least three molecular species (S1, mol wt 31,500 and S2, mol wt 54,700, the main types). Type S, urokinase had a specific activity approximately twice that of S2, but antisera prepared against S, showed reaction of identity with S, material.…”