A 30-kDa protein in the medium of cultured human umbilical vein endothelial cells (HUVECs) was identified as (pro)renin receptor, (P)RR, by western blot analysis using anti-human (P)RR antibodies. The protein bound recombinant human prorenin with a K(D) of 4.0 nmol l(-1) and activated prorenin. These observations suggest the presence of soluble (P)RR, s(P)RR, in the medium of cultured HUVECs. For quantification of the s(P)RR in the medium, an enzyme-linked immunosorbent assay (ELISA) was established. The quantitative range of the ELISA was validated over a nominal range of 7.5-300 pmol l(-1) in the wells of a microtiter plate. The assay system showed good linearity (r(2)=0.99) with interassay (5.8-9.7%) and intraassay (2.1-7.0%) precision. Using this method, the concentration of s(P)RR in the culture medium of HUVECs was measured to be 32 pmol l(-1). Therefore, these results show qualitative and quantitative evidence that prorenin can be activated after binding to s(P)RR secreted from cultured HUVECs.