A normal human lymphocyte-derived 54 kDa polypeptide, capable of regulating cell growth has been identified as an isoform variant (abbreviated as NP54) of protein neuroleukin-phosphoglucose isomerase (PGI). Since distinct PGI variants undetectable in normal tissues had been identified in breast cancer tissues, the effect of NP54 on the growth of human breast cancer cells SK-Br-3 in cultures was analyzed. Exposure to NP54 caused a dose-dependent growth modulation. Approximately 40% reduction of cell density was detected at 40 pM of NP54, along with a blocking of G1 cells entering into S phase. The growth modulation was correlated with a significantly reduced expression of epidermal growth factor receptor (EGF-R) gene transcript, supporting the interpretation that the level of EGF-R expression and cell growth are related mechanisms. NP54 treatment also significantly increased cells with apoptotic morphological feature and fragmented DNA. Incubation with a monoclonal anti-NP54 antibody negated NP54 activity, confirming a regulatory activity in cell growth and apoptosis.