A fast and selective analytical method, used to determine the different lysophosphatidic acid (LPA) species in serum, has been developed and validated. LPA species were quantitatively extracted from serum using methanol-chloroform (2:1, v/v). The proteins were precipitated by this solvent mixture and separated by centrifugation in one step. LPA levels were determined in clear extracts using the HPLC-MS/MS method. The linearity of this method was established in the concentration range between 0.1 and 16 microM for all LPA species with a correlation coefficient greater than 0.99. Recovery of all LPA species determined by the serum, fortified at approximately 1 microM and 2-3 microM, was between 93% and 111% with an average R.S.D. of less than 8%. This method was used to determine LPA in numerous sera of healthy controls, patients with benign ovarian tumours and ovarian cancer at different stages. Significantly higher total LPA levels were determined in the sera of patients with different types of tumours (benign and malignant).