Fifteen unrelated individuals were found among the patients of the Cytogenetics Laboratory who possessed multiple-satellited marker chromosomes (14 with double satellites and 1 with triple satellites). Cytogenetic analysis was carried out by means of a conventional staining method and also by R, C, and Q banding and by the technique of silver staining. The intercalar structures of all 15 cases differed from the terminal satellites in their biochemical composition: they were resistant to heat denaturation, and stained heavily with the R-banding technique. Accordingly, they consisted of GC-rich material identical with that which in varying quantity is a regular constituent of the short arms of acrocentric chromosomes. The findings described indicate that any larger accumulation of such R band-positive material tends to dissociate from the basal segment of short arms by a proximal secondary constriction. We therefore assume that the formation of intercalar satellites may be interpreted as a cytological consequence of extreme natural R polymorphism.