This paper describes the enzyme catalyzed synthesis of the undecapeptide substance P from its non-associated fragments (1-7) and (8-11) or (1-8) and (9-11). The fragment condensation was mediated by the use of product specific antibodies as molecular traps. As catalyst a previously purified endopeptidase was used which specifically hydrolyzes substance P at the Phe7-Phe8 and Phe8-Gly9 bonds. The synthesis was performed in analytical scale and product formation was guided by reversed phase HPLC combined with radioimmunoassay. It appeared that the substance P fragments (1-8) and (9-11) were condensed to a larger extent than (1-7) and (8-11). This observation may well result from the higher affinity of the antibodies observed for substance P (8-11) as compared to that found for the other fragments. Increased concentration of the antibodies also seemed to result in enhanced resynthesis of substance P.