The fine structure and distribution of lymphatics in the synovial membrane of monkey knee joints, and human knees with osteoarthritis and rheumatoid arthritis, were studied by light and electron microscopy using an enzyme-histochemical method. Whole mount preparations and tissue sections were doubly stained for 5'-nucleotidase and alkaline phosphatase. The 5'-nucleotidase positive lymphatics could be distinguished from the alkaline phosphatase positive blood vessels. In monkey specimens the 5'-nucleotidase lymphatics were densely distributed in the medial and lateral parts of the suprapatellar pouch and infrapatellar fat pad, in both the superficial and deep of the sublining cell layers. Collecting lymphatics were well developed in the deep fibrous layer and in the merging epimysium. In the knees with both types of arthritis, the patterns of distribution were the same as in the monkey. No lymphatics were found in well developed villi in rheumatoid arthritis, but many blood vessels were present.