The pattern of serum proteins separated by SDS-PAGE is typified by the microprotein apolipoprotein A I which is split from high density lipoprotein by SDS. High density lipoprotein is generally retained by the glomerulus and does not appear in the urine even in glomerulopathies. Thus, apolipoprotein A I is generally absent in the SDS-PAGE protein pattern of renal proteinurias. However, in postrenal hematurias and proteinurias apolipoprotein A I can be found by SDS-PAGE, immunoblotting and Ouchterlony test. Using rabbit anti-human-apolipoprotein A I as primary antibody and alkaline phosphatase-conjugated anti-rabbit immunoglobulins as secondary antibody antibody apolipoprotein A I could be detected even at a 1:128,000 dilution of blood. This means a microhematuria of only 8 microliters blood/l urine theoretically can be identified as postrenal. Unfortunately, apolipoprotein A I is not only visible on the immunoblots of postrenal hematurias and proteinurias but could also be seen in renal proteinurias. Thus, only with reservation can apolipoprotein A I be called a marker of postrenal hematuria and proteinuria. On the other hand, most renal proteinurias can be identified reliably by SDS-PAGE and analysis of apolipoprotein A I is superfluous. Apolipoprotein A I, however, could become useful in the differentiation of microhematurias without proteinuria.