Methods for evaluating dialyzer clearance of beta 2-microglobulin during clinical hemodialysis have not been well established. The authors show, theoretically, that the postdialysis-to-predialysis concentration ratio, a parameter often used to estimate dialyzer clearance of beta 2-microglobulin, depends on KdT/V (the dialyzer clearance times the treatment time divided by the distribution volume for beta 2-microglobulin) and the ultrafiltration rate, assuming that a single compartment kinetic model is valid. They also show that adjustment of the postdialysis concentration of beta 2-microglobulin for changes in its volume of distribution does not entirely correct for fluid removal when the adjusted postdialysis-to-predialysis concentration ratio is significantly below one. These considerations suggest that estimates of dialyzer clearance of beta 2-microglobulin using single compartment models are more reliable than those using only the postdialysis-to-predialysis concentration ratio. To illustrate these constructs, the authors compared experimental estimates of beta 2-microglobulin clearance during clinical hemodialysis using single compartment models with those measured directly from the arteriovenous concentration difference across the dialyzer. First-use low flux and high flux-dialyzers and those reprocessed with Renalin were studied. Single compartment estimates of beta 2-microglobulin clearance for low flux dialyzers were similar to those measured directly across the dialyzer, but single compartment estimates of beta 2-microglobulin clearance for high flux dialyzers exceeded (p < 0.001) those measured directly across the dialyzer, independent of whether fluid removal during hemodialysis was assumed to be removed entirely from the extracellular compartment or proportionally from both intracellular and extracellular compartments. The authors conclude that accurate estimates of beta 2-microglobulin clearance for high flux dialyzers will require kinetic models that are more complex than those assuming a uniform distribution of beta 2-microglobulin in a single, well-mixed compartment.