PURPOSE: To provide a soluble peptide which resembles the chemotaxis factor of a specific peritoneum fluid, has a cellular infiltration prevention capability, and is useful for diagnosing or treating endometriosis.

CONSTITUTION: A soluble peptide is provided which has a mol.wt. (determined by separation with a G75 column) of about 15 KD to about 20 KD, can cause neutrophil and macrophage chemotaxis, and resembles the chemotaxis factor of a peritoneum fluid of a mammal suffering from endometriosis of a slight to medium degree. Endometriosis of a mammal is diagnosed or treated with an antibody specific to the soluble peptide, a natural chemotaxis factor. The antibody is selected from among monoclonal antibodies, polyclonal antibodies, Fab fragments, and chimera antibodies and is used as a soln. or an antibody preparation fixed on a solid support.