middle molecule (mm) fractions (7a, b, c) obtained by chromatography were each separated in at least two fractions with different mobility and uv-absorption. preparative isotachophoresis proves to be a suitable method for isolation of pure mm compounds. the automatic mm analyzer, which combines gel filtration and ion exchange gradient elution chromatography was used for analysis of mm. the results together with various clinical and biochemical data are stored and processed by a pdp 8 computer. non-dialyzed and dialyzed patients are now followed on a prospective basis and correlations are sought to residual renal function, efficiency of dialysis, biochemical variables and clinical symtoms. the results are not fully evaluated yet. hemoperfusion through active carbon removed mm from uremic blood in vivo, but was not more effective than hemodialysis. mm exerted toxic effects in vitro on erythropoiesis, granulocyte stem-cell proliferation, platelet aggregation and on the enzyme gamma-laevuliniv acid hydrase, but no on single nerve fibers