Differential leukocyte counts on noninflammatory synovial fluids (NISF) are not widely reported or used in research, apparently due to technical difficulties related to either high viscosity or low numbers of cells. We describe an evaluation of a technique using hyaluronidase and cytospin preparations to study NISF. Twenty-three consecutive synovial fluids (SF) with less than 2,000 white blood cells (WBC)/mm3 were studied either by the usual smear of a single drop or by adding two drops of hyaluronidase (150 USP units/ml) to 0.25 cc of SF and cytocentrifuging at 800 rpm for 10 min. Both preparations were stained with Wright's stain. Cytospin preparations gave better morphology, and in 22/23 specimens we could count 100 cells on one slide. Smeared preparations gave dark cells and required 2-3 slides to count 100 cells. Differential counts on the cytospin preparations consistently showed higher percentages of monocytes, suggesting that these cells were underdetected and misinterpreted as lymphocytes on the routine smears. Polymorphonuclear leukocytes (PMN) were significantly less frequent (P 0.005) in osteoarthritis (OA) fluids than in the other diseases with NISF. Relatively more PMN may suggest consideration of a diagnosis other than OA. Cytospin preparations of hyaluronidase-treated NISF may open up an important area for investigation of the role of SF cells in less inflammatory diseases.