The objective of the current study was to measure the friction coefficient simultaneously with the interstitial fluid load support in bovine articular cartilage, while sliding against glass under a constant load. Ten visually normal 6-mm-diameter cartilage plugs harvested from the humeral head of four bovine shoulder joints (ages 2 4 months) were tested in a custom friction device under reciprocating linear motion (range of translation +2 mm; sliding velocity 1 mm/s), subjected to a 4.5 N constant load. The frictional coefficient was found to increase with time from a minimum value of pmln = 0.010 f 0.007 (mean f SD) to a maximum value of 0.243 k 0.044 over a duration ranging from 920 to 19,870 s (median: 4,560 s). The corresponding interstitial fluid load support decreased from a maximum of 88.8 k 3.8% to 8.7 5 8.6%. A linear correlation was observed between the frictional coefficient and interstitial fluid load support (r2 = 0.96 * 0.03). These results support the hypothesis that the temporal variation of the frictional coefficient correlates negatively with the interstitial fluid load support and that consequently interstitial fluid load support is a primary mechanism regulating the frictional response in articular cartilage. Fitting the experimental data to a previously proposed biphasic boundary lubrication model for cartilage yielded an equilibrium friction coefficient of peq = 0.284 f 0.044. The fraction of the apparent contact area over which the solid cartilage matrix was in contact with the glass slide was predicted at cp = I .7 i 6.30/,, significantly smaller than the solid volume fraction of the tissue, cp' = 13.8 i 1.8%. The model predictions suggest that mixed lubrication prevailed at the contact interface under the loading conditions employed in this study.