A specific marker involving probably a #10 chromosome was present in 4 of 5 Burkitt lymphoma lines (EB1, EB2, EB3, and JIYOYE). The marker consisted of a subterminal secondary constriction or achromatic gap in the long arms of 1 or both #10 chromosomes. It is proposed that this lesion defines a specific abnormal chromosome. The fifth cell line investigated, SL1, had a translocation which possibly could have also involved the #10 chromosome. All these cell lines were either diploid, pseudodiploid, or near-diploid. None were highly myxoploid. An extra C chromosome was noted in the near-diploid JIYOYE cell line, and the appearance of cells with an extra chromosome in some of the population was observed in several other cell lines (EB1, EB2, and SL1).