Prompted by Castor's investigations (4) on a connective-tissue-activating peptide (CTAP) we investigated the stimulative effect of synovial fluids and synovial-tissue extracts on the synthesis of collagen by incubated embryonic-chick tendon cells. The stimulative effect was greater with synovial-tissue extracts from more severe cases than with samples from patients having milder forms of the disease. There was no correlation between the stimulation by synovial fluids and synovial-tissue extracts from the same patient. The stimulative activity was lost at dialysis. A slight stimulation in the incorporation of glucosamine was also observed. Treatment of the patients with gold, chloroquine or steroids decreased the stimulating capacity. These effects seem to depend on factors different from those described by Castor. The differerences in the stimulant activity of samples from various groups cannot be entirely due to glutamine, which is one of the limiting nutrients of the embryonic-chick tendon cells.