1. Gel electrophoresis of proteoglycans extracted with use of 4 M-guanidinium chloride from baboon (Papio papio) articular cartilage and purified on DEAE-cellulose in 8 M-urea yielded three bands on electrophoresis in polyacrylamide/agarose gels: two wide bands close together (I and II) and a third, thinner and more rapidly moving band (III). 2. Gel electrophoresis of fractions from direct ‘dissociative’ gradients showed that these bands were partially separated (buoyant density of I greater than II greater than III). 3. Reduction and alkylation of proteoglycans did not alter either the gel-electrophoretic pattern or the distribution of the bands in the fractions of the gradient. 4. Band III was found in the upper third of ‘associative’ gradients but not in the bottom fraction, which yielded after dissociation only bands I and II. 5. The third band was completely extracted for 24h with an iso-osmotic solution, but was contaminated with bands I and II. The second extraction step with 4M-guanidinium chloride yielded only bands I and II. 6. The data strongly suggest the presence in the articular cartilage of several populations of dissociated proteoglycans differing in gel-electrophoretic migration, buoyant density and aggregation capacity.

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