Highly inbred CBA mice were used. The registration of chromosome abnormalities was limited to numerical deviations and the occurrence of metacentric chromosomes. By separate serial transplantation from a 90Sr-induced osteosarcoma two parallel transfer series (B and b) were established. From these series transplantation was also performed to hyperimmunized hosts B (Hi) and b (Hi). Besides differences in mean outgrowth period between B-and b-generations, a variation in chromosome pattern was observed. However, this variation should not be evaluated as a typical chromosomal progression of fast and slow growing tumour series.