1. An activator of leucocyte latent collagenase has been extracted from rheumatoid synovial fluid by a preparative method consisting of six steps including precipitation by ammonium sulphate and chromatography on Sephadex G-100, QAE-Sephadex and SP-Sephadex C-50. The purification factor was nearly 1000 and the activator isolated could be shown to have a high degree of homogeneity.--2. Gel chromatography indicated a molecular weight of ca. 60 000.--3. Kinetic studies of the activation and inactivation of the activator during incubation at higher temperatures demonstrated its enzymic nature.--4. Activation of latent collagenase was partially inhibited by iPr2P-F and KCN. Soybean trypsin inhibitor, iodoacetamide, TosLysCH2Cl and TosPheCH2Cl had no effect.--5. Leucocyte latent collagenase was also activated by an excess of trypsin and p-hydroxymercuribenzenesulphonic acid, but only to the extent of about 40% of its activation capacity. Purified neutral protease from human leucocyte granules had no effect on latent collagenase.--6. Several typical substrates for proteases, peptidases, esterases and glycosidases were not attacked by the activator. The possibility that the activator is a known enzyme, such as kallikrein, urokinase or cathepsin B1, could be excluded.