It is generally recognized that the partition between plasma and blood cells, the immediate centrifugation of blood samples after collection for the measurement of 'true' in vivo concentrations and free drug concentrations in plasma are important determinants of the pharmacokinetics and/or pharmacodynamics of drugs. Therefore, the stability, blood partition between plasma and blood cells, and factors influencing the binding of ipriflavone to 4% human serum albumin (HSA) using an equilibrium dialysis technique were evaluated. Ipriflavone was unstable in rat liver homogenate and various pH solutions ranging from 1 to 13, except pH 8, rat blood and plasma and human plasma when incubated in a water-bath shaker for 24 h kept at 37ยฐC and at a rate of 50 oscillations/min. The recoveries of spiked amounts of ipriflavone at 24 h pH solutions ranging from 1 to 12 were 67.0, 78.1, 87.9, 89.6, 84.2, 87.4, 85.5, 99.3, 88.0, 76.6, 79.4 and 81.5%, respectively. Ipriflavone was very unstable in pH 13 solution; only 0.814% of ipriflavone was recovered after 30 min incubation. Ipriflavone was stable for up to 3 h incubation in human gastric juices. Ipriflavone reached equilibrium fast (within 30 s of being mixed manually) between plasma and blood cells and the equilibrium plasma/blood cells partition ratios were independent of the initial rabbit blood concentrations of ipriflavone: 0.2, 2, and 10 mg/mL; the values were in the range of 0.900-2.45. The binding of ipriflavone to 4% HSA was 96.69 0.407% at ipriflavone concentrations ranging from 2 to 100 mg/mL, but it was dependent on HSA concentrations (0.5 -6%), incubation temperature (4, 22 and 37ยฐC), 'the buffer' pHs (5.8, 6.4, 7.0, 7.4 and 8.0), and addition of salicylic acid (150 -300 mg/mL) and sulphisoxazole (100-300 mg/mL). However, the binding was independent of buffers containing various concentrations of chloride ion (0-0.546%), glucose (0 and 5%), alpha-1-acid glycoprotein (0 -0.32%) and heparin (0-40 U/mL), and addition of its metabolites (M1 and M5, 5 mg/mL).