Determination of fetal red blood cell antigens in early pregnancy can be important in cases with a history of severe haemolytic disease of the newborn. From chorionic villus biopsies (CVB) between the 8th and 12th week of gestation a small number of fetal red blood cells was obtained, inevitably highly contaminated with maternal blood cells. Two techniques were used to demonstrate the minor (fetal) cell population with a blood group antigen differing from the major (maternal) cell population: a solid-phase microfluorescence technique (introduced in this paper) which was compared with the mixed agglutination technique. In series of artificial mixtures of erythrocytes it was shown that with the microfluorescence technique the ABO and Rhesus phenotypes of minor cell populations could be determined at a ratio of 1 in 4000 erythrocytes of the major population, making this technique 4 times as sensitive as the mixed agglutination technique. We further investigated the reliability of the microfluorescence technique to demonstrate antagonistic fetal blood groups in the first trimester of pregnancy. Of 18 women undergoing CVB prior to therapeutic abortion, blood group antagonism (ABO and Rhesus systems) was demonstrate in all 11 cases in which it was present. Therefore, it seems that CVB can be reliably used for the prenatal diagnosis of (recurrent) blood group antagonism.