This report deals with the effect of purified HSPI (10.7 kDa single chain prostatic peptide of 94 amino acids) on PHA-stimulated proliferation of peripheral blood mononuclear cells (PBMNC). HSPI was found to inhibit the incorporation of 3H-thymidine by PHA-stimulated PBMNC in a dose-dependent manner, albeit with the requisite of pretreatment of cells with HSPI. A maximum inhibition was observed on pretreatment for 30 min. These studies thus indicate that HSPI may be interfering with the very early events of PHA-induced signal transduction process in PBMNC.