Synthesis of peptides related to hydra head activator (HA; pGlu-Pro-Pro-Gly-Gly-Ser-Lys-Val-Ile-Leu-Phe-OH) was performed by the solution method. The analogs synthesized were Tyr11-HA, des-Phe11-HA, Arg1-HA and Phe5-HA. Antisera were prepared by immunizing three rabbits with the synthetic HA. Two anti-HA-antisera with sufficient titer were obtained. An HA-radioimmunoassay system, developed by using an antiserum (ASH-04) and 125I-Tyr11-HA as a tracer, was found to be sensitive and specific to HA.