We have developed a sensitive and quantitative RT-PCR assay for the determination of tissue factor (TF) mRNA levels in human cells. An in vitro synthesized internal standard RNA was used to correct for differences in reverse transcription or amplification of various RNA samples. The PCR products were quantitated by hybridization. The sensitivity was such that less than 0.2 microgram of total endothelial RNA sufficed to measure its TF mRNA content. The RT-PCR assay was used to determine TF mRNA levels in endothelial cells treated with a factor from human melanoma cells and/or TNF. In this way the amount of TF mRNA could be induced to a level that was at least 80-fold higher than that in non-induced cells. This increase was in the same order of magnitude as the induction of measured TF activity.