This experimental study was undertaken to clarify whether beta 2-microglobulin (B2M) deposits are the cause or merely the consequence of arthritis or osseous changes in chronic renal failure. Twenty-one heminephrectomized, male inbred DBA/1J mice were divided into 3 groups of 6 animals and a group of 3 mice. Collagen arthritis was produced by the intradermal injection of an emulsion of bovine collagen type II and complete Freund's adjuvant in groups 1, 2 and 4. Human urine derived B2M, 100 micrograms and 300 micrograms was then given subcutaneously to group 1 for up to 10 days and also to the 6 mice in group 3 without arthritis. Three mice in group 4 received horse myoglobin 100 micrograms daily for 1, 3 and 5 days as a control. Regardless of the presence of arthritis or osseous changes, B2M was deposited predominantly in the bone marrow and synovium of the B2M treated mice in groups 1 and 3. On the other hand, group 2 which had not been given B2M had no deposits. Deposits of myoglobin were not seen in group 4 mice. These findings suggest that B2M is deposited in the bone marrow and synovium as a primary event.