We have investigated the effects of phospholipids on activation and proliferation of ovarian and breast cancer cells. Lysophosphatidic acid (LPA), lysophosphatidylserine (LPS) and sphingosylphosphorylcholine (SPC) all induce transient increases in cytosolic free Ca2+ ([Ca2+]i) in both ovarian and breast cancer cell lines. The ability of LPA, LPS and SPC to induce increases in [Ca2+]i in ovarian and breast cancer cells is likely to be due to an interaction with cell-surface receptors as the increases in [Ca2+]i were: (1) due to release of calcium from intracellular stores and not from transmembrane uptake due to changes in permeability; (2) blocked by lanthanum and suramin which do not enter cells; (3) blocked by phorbol esters which interrupt increases in [Ca2+]i induced through a number of different receptors; and (4) not detected in freshly isolated peripheral blood mononuclear cells, indicating cell type specificity. In addition, increases in [Ca2+]i induced by LPA, LPS and SPC in ovarian and breast cancer cells completely self-desensitized and cross-desensitized each other, but did not block increases in [Ca2+]i induced by thrombin. Lysophosphatidylglycerol (LPG), but not other lysophospholipids, inhibited LPA- but not LPS- or SPC-induced increases in [Ca2+]i, suggesting that LPA may interact with a different receptor(s) to LPS or SPC and that their downstream signalling pathways converge or interact. LPA, SPC and LPS also induced rapid increases in tyrosine phosphorylation of specific cellular proteins, including p125FAK. Strikingly, LPA, but not LPS or SPC, induced activation of mitogen-activated protein (MAP) kinases. Despite an ability to activate similar intracellular signaling events, LPA, LPS and SPC exhibited markedly different effects on cell proliferation. Whereas LPA induced a significant increase in cell proliferation, LPS did not substantially alter cell proliferation and SPC inhibited cell proliferation. Surprisingly, phosphatidic acid (PA), which did not induce increases in [Ca2+]i, p125FAK activation or activation of MAP kinases, did induce proliferation of ovarian cancer cells, albeit at higher concentrations that LPA. The discordance between sensitivity to LPG, early biochemical events stimulated, and the eventual proliferation response combine to suggest that LPA probably utilizes a different receptor from LPS, SPC and PA. Therefore ovarian and breast cancer cells are sensitive to the effects of a number of different phospholipids which may play a role in the growth of these tumour cells in the cancer patient and are thus potential targets for therapy.