Urine inhibition of calcium oxalate monohydrate (COM) crystal growth (CG) seems due to a glycoprotein that contains gamma-carboxyglutamic acid and has been named nephrocalcin (NC); however, Tamm-Horsfall protein (THP) and albumin resemble NC and make its measurement and role uncertain. NC in urine is aggregated to molecular mass 64 kDa and higher, similar to albumin (64 kDa) and THP (87 kDa). Albumin and THP are calcium binding, albumin adsorbs to COM crystals, and THP has been described as an inhibitor of COM growth. Antisera to NC have cross-reacted with THP even though the NC was isolated from cultured renal cells. Here we have compared highly purified NC, THP, and albumin adsorption with COM crystals and CG inhibition; also we compared their patterns of cross-reactivities with a new antiserum against NC and a monoclonal antibody to THP. NC adsorbs to COM crystals, THP does not. Albumin and THP do not inhibit CG. Cross-reactivity of albumin and THP to the antiserum is slight by direct enzyme-linked immunosorbent assay and nonexistent by competitive ELISA; reaction of NC to the anti-THP monoclonal antibody is absent.