This invention provides methods for detecting the presence of malignant or premalignant cells, or trophoblastic cells in a human wherein the malignant, premalignant or trophoblastic cells express 5T4. The methods of the invention detect 5T4 RNA in blood, blood plasma, serum, and other bodily fluids. The inventive methods are useful for detection, diagnosis, monitoring, treatment, or evaluation of neoplastic disease, and for the detection and evaluation of placental tissue in pregnant women.