Fetal blood sampling under ultrasound control is rapidly expanding the study of human fetal biology. Pure fetal blood is required for prenatal diagnosis, establishment of reference ranges for biologic measurements, and assessment of fetal welfare. We present here the methods that we have developed to detect contamination in more than 1500 samples. These include hematologic indexes, blood smear, erythrocyte antigens, beta-human chorionic gonadotropin, and coagulation factor assays. The tests are relatively simple, inexpensive, and widely available. No single test is reliable in all situations, and it is necessary to perform all the tests on each sample of fetal blood. In the clinical setting where irrevocable action may be taken as a consequence of our results, we require absolute assurance of the purity of the sample.