Chromosome studies were performed on 5 Ewing sarcoma cell lines. An identical reciprocal translocation t(11; 22) (q24; q12) was found in 4 cell lines established from 3 different tumors. These results, associated with those obtained at the same time and independently from fresh tumor cells, suggest that the translocation t(11; 22)(q24; q12) may be a chromosomal marker characteristic of Ewing sarcoma cells. This translocation involves the chromosome 22 on which the H-c-sis oncogene has been located; it could be used as a new tool for exploring the role of genetic transposition in the malignant cell transformation.