Serum hyaluronate was monitored in volunteer patients with rheumatoid arthritis who were mobilized on day 1 at 7 am, but stayed at rest in bed on day 2 until mobilized at 11:30 am. 125I-hyaluronate binding protein assay disclosed a normal morning peak on day 1 but not on day 2. Instead, on day 2 transiently high hyaluronate levels were seen after 11:30 am. This suggests that joint mobilization affects serum hyaluronate. The morning peak may depend on the accumulation of hyaluronate in immobilized joints at night and its mobilization in the morning. High performance liquid chromatography with a size exclusion column used for fraction collection with 125I-hyaluronate binding protein assay showed that the serum hyaluronate in the morning peak is of relatively low molecular weight when compared to synovial fluid hyaluronate. This difference may be due to degradation of synovial fluid hyaluronate before its clearance into the blood stream.