Boundary lubricating activity and biochemical characteristics of purified lubricating factor from bovine synovial fluid (PSLF) were compared to those of mucinous glycoprotein from human submandibular saliva and stimulated tears. Mammalian synovial fluid and saliva contain mucinous glycoproteins which reduce the coefficient of friction (mu) in a bearing of latex:glass which isolates boundary lubrication. In contrast, mucin secreted by the lacrimal gland did not lubricate. Cleveland plotting showed that these species are not identical.