Chromosome studies on 26 cases of Burkitt's cell malignancy are reported, including 7 lymphomas and 19 acute lymphocytic leukemias. A specific translocation was found in 26 of the patients: 20 t(8;14), 4 t(8;22), and 1 t(2;8). In one other case only 2 mitoses out of 59 were found to have a probable t(8;22) translocation. Additional chromosomal abnormalities were found in 15 of the patients, irrespective of whether they had or had not been treated before cytogenetic investigation. The most remarkable was a partial duplication of chromosome 1q. The presence of additional chromosomal abnormalities together with a specific translocation allows a distinction to be made between specific and secondary chromosome anomalies. The implications of the two types of aberrations for research into the molecular mechanisms of malignancy are discussed.