Aliquots from 30 synovial fluids were submitted to 4 laboratories for comparison of leukocyte counts and differential cell counts, and to 3 laboratories for a search for and identification of crystals. Leukocyte counts showed only fair correlation (coefficients of 0.76-0.80) with the reference laboratory. In synovial fluid from 4 patients, there was sufficient difference in leukocyte counts to cause the fluids to be erroneously classified as either "inflammatory" or "noninflammatory". In 12 of 24 fluid specimens examined, percentages of neutrophils fell outside the 95% confidence limits of the value determined by the reference laboratory. In 7 of the 11 patients with crystals reported, discrepancies were found between the reports from 1 or more laboratories. More attention to quality control of synovial fluid analyses is important.