A novel phospholipase A.sub.1 (PLA.sub.1) having a substrate specificity to phosphatidic acid (PA); a peptide or a polypeptide originating in the above novel PLA.sub.1; a polynucleotide encoding the peptide or polypeptide originating in the novel PLA.sub.1; a process for producing the peptide or polypeptide originating in the novel PLA.sub.1; an antibody against the peptide or polypeptide originating in the novel PLA.sub.1; a method of identifying an inhibitor, an antagonist or an activator of the novel PLA.sub.1 by using the same; compounds identified by the above method; and medicinal compositions and diagnostic methods by using the same.