Cells derived from Bax-null muscles immortalized more rapidly than cells from Bax-positive muscles. Primary cultures of Bax (-/-); Bax (+/-); Bax (+/+); and p53 (-/-) myogenic cells were established, serially passaged, and enumerated at each passage as described in Materials and Methods. Results of three independent experiments are shown. As described in the text, myogenic cells from Bax-null muscles (closed circles) immortalized more rapidly and more frequently than myogenic cells from either Bax (+/-) heterozygous muscles (crossed circles) or Bax (+/+) wild-type muscles (open circles). However, myogenic cells from p53 (-/-) muscles (stars with dotted line in Expt. 3) immortalized faster than Bax-null cells without showing the temporary plateau in growth rate shown by Bax-null cells. Each symbol denotes a time at which cells were collected and passaged onto new culture dishes as described in Materials and Methods.