Stanniocalcin (STC) decreases branchial Ca(2+)-uptake in fish. In order to determine its bioactive domain, synthetic fragments (U amino acids (aa) 1-20; V aa 103-136; W aa 202-231) of eel STC were tested for their effect on Ca2+ uptake in tilapia (Oreochromis mossambicus). Ca2+ uptake was inhibited by an N-terminal fragment but not by a midfragment nor a C-terminal fragment of the mature hormone. We provide theoretical and experimental evidence that a midportion of STC, which is included in the synthetic fragment V, is the most antigenic site of the molecule. Polyclonal antibodies against stanniocalcin are directed against this midportion although this region of STC appears not to be essential for signal transduction. These results suggest that the currently available antibodies will recognize inactive STC fragments in the circulation. We conclude that the bioactive portion of STC does not correspond with the major antigenic portion of the hormone. The results imply that studies on plasma STC levels employing a polyclonal antiserum against STC should be interpreted with care.