Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is a highly specific enzyme whose only known substrate is the GPI anchor of cell surface proteins. GPI-PLD measurements, however, are technically difficult since the enzyme is expressed at low levels in cells and tissues, and serum contains large amounts of inactive, latent GPI-PLD interfering with protein-based assays. We have therefore developed a semi-quantitative RT-PCR method to measure mRNA expression of all known GPI-PLD isoforms in cells and tissues. In human ovarian cancer cell lines, GPI-PLD mRNA expression correlated with GPI-PLD enzyme activity and with the shedding of the GPI-anchored tumor and prognostic markers, urokinase receptor and CA125, from the cell surface. This supports a potential role for this enzyme in the generation of circulating prognostic markers in malignant tumors. Similarly, in human epithelial cells of the skin, GPI-PLD mRNA expression increased with tumor progression. Whereas normal keratinocytes did not express significant amounts of GPI-PLD mRNA, expression was dramatically induced by serum in immortalized HaCaT keratinocytes and constitutively high and independent of serum in tumorigenic A431 epidermoid carcinoma cells. In addition, GPI-PLD expression was significantly increased in highly malignant. H-ras-transfected murine bladder carcinoma cells as compared to the low malignant, non-transfected parental cells. The competitive RT-PCR described here represents the first quantitative assay specific for cellular GPI-PLD isoforms, and our in vitro analyses suggest that GPI-PLD expression might be associated with tumor malignancy.