One hundred normal American Blacks were studied by q-banding technique in order to estimate the type and frequency of heteromorphisms in the centromeric regions of chromosomes 3 and 4. Heteromorphisms were classified using the scale of five intensity levels namely, negative, pale, medium, intense and brilliant fluorescence. QFQ intensity heteromorphisms (greater than or equal to level 3) for chromosomes 3 and 4 were 54.5 and 7%, respectively. The distribution of different intensities was significantly different in American Blacks compared with Caucasians.