The association between the brush border enzyme alkaline phosphatase and gamma-glutamyltransferase was determined by sucrose density gradient analysis of crude kidney homogenates, isolated glomeruli, and isolated microvessels. As previously established there is an overlap of these enzyme activities in the crude homogenate corresponding to a density of 1.17 g.cm-3. In contrast, isolated glomeruli sedimented with a peak of 1.25 g. cm-3 and exhibited gamma-glutamyltransferase activity but little alkaline phosphatase activity; homogenizing isolated glomeruli shifted the fragments to a density coincident with that observed for the crude homogenate gamma-glutamyltransferase peak. A second population of capillaries, isolated microvessels, were homogenized and analysed on the sucrose density gradient. These fragments sedimented over the same range as crude homogenate gamma-glutamyltransferase peak but were devoid of alkaline phosphatase activity and yet exhibited remarkable gamma-glutamyltransferase activity. The results indicate homogenization of renal cortex results in a heterogeneous collection of particles from both tubular and microvascular locations exhibiting gamma-glutamyltransferase activity which overlap with the brush border alkaline phosphatase containing membranes. However, isolation of microvessels and glomeruli prior to homogenization allows separation of gamma-glutamyltransferase from alkaline phosphatase activity; between 10 and 20% of the total homogenate gamma-glutamyltransferase activity is estimated to be associated with the microvascular compartment.