LPA (lysophosphatidic acid) is a bioactive phospholipid having diverse effects on various types of tissues. When NMuMG (normal murine mammary gland) cells were cultured in the presence of 0–10 μM LPA, cell numbers were increased by dose dependency for the 6-day culture periods (P<0.05). In DNA synthesis assay, 10 μM LPA induced 4.5-fold more DNA synthesis compared with control (P<0.05). In addition, the cultured cell density in the given area was increased by LPA treatment. MMP (matrix metalloproteinase) inhibitor GM6001 and EGFR [EGF (epidermal growth factor) receptor] tyrosine kinase inhibitor AG1478 [tyrphostin AG1478, 4-(3-chloroanilino)-6,7-dimethoxyquinazoline] significantly decreased LPA-induced DNA synthesis and cell growth without cell death (P<0.05). To test the hypothesis that LPA-induced cell growth is mediated through LPA subtype receptors, LPA subtype receptor gene expressions were amplified by PCR. NMuMG cells expressed LPA1 and LPA2 receptor genes in the presence of 10% FBS (fetal bovine serum). LPA treatments increased ERK1/2 (extracellular-signal-regulated kinase) phosphorylation at 30 min and then dephosphorylated at 2 h after treatment. LPA treatment phosphorylated at tyrosine residues on a variety of Gi and PI3-dependent signal transducers in NMuMG cells. These results suggest that LPA subtype receptors play a role as the active transactivator of EGFR-associated kinases as well as direct growth regulator in mammary tissues.