The principles governing solute flux or transport in different artificial kidney treatment modalities are reviewed. Solute clearance profiles were calculated for identical artificial kidney membranes during haemodialysis, haemofiltration and haemodiafiltration. It was shown that the clearance of small solutes depends largely on the dialysate flow rate and is similar when using either haemodialysis or haemodiafiltration. In contrast, clearance of middle molecules, especially low-molecular-weight proteins, depends largely on convective transport induced by high ultrafiltration rates and is maximized when using either haemofiltration or haemodiafiltration. Optimal fluxes for both small solutes and middle molecules can be achieved by using postdilution haemodiafiltration. Recent work has shown that use of the reduction in plasma concentration, even after normalization for changes in extracellular volume during therapy, is not an exact measure of beta2-microglobulin (and other low-molecular-weight proteins) clearance. It is proposed that beta2-microglobulin clearance be reported in future studies instead of the normalized reduction in beta2-microglobulin plasma concentration. Additional studies are necessary to determine the effects of postdialysis rebound on the calculated clearance for beta2-microglobulin and other high-molecular-mass uraemic toxins.