Gains and losses of chromosomes or chromosome arms were recorded in 45 of 165 human B-cell lines. Most aberrations were acquired in vitro, and their frequency was related to duration of culture. Gains occurred more frequently than losses and their distribution was nonrandom. Chromosomes most commonly affected were No. 3, 7, 8 (particularly 8q), 9, 12, and 21. Certain differences in the frequency of particular aberrations appeared to be related to the clinical conditions of the patients from whom the lines were derived. The distribution of chromosome gains in this material was correlated with those detected in direct preparations from human tumors.