The function of prothymosin alpha has been investigated by using four different antisense oligodeoxyribonucleotides directed at selected regions of its mRNA. In every case, when synchronized human myeloma cells were released from stationary phase by incubation in fresh medium containing antisense oligomers, cell division was prevented or inhibited; sense oligomers and random antisense oligomers had no effect. A detailed analysis of synchronized cell populations indicated that sense-treated and untreated cells divided approximately 17 hr after growth initiation, whereas cells incubated with antisense oligomer 183, a 16-mer targeted 5 bases downstream of the initiation codon, entered mitosis approximately one cell division late. The failure to divide correlated directly with a deficit in prothymosin alpha and with the continued presence of intact intracellular antisense oligomers over a period of at least 24 hr. Because antisense oligomers had no effect either on the timing of the induction of prothymosin alpha mRNA upon growth stimulation or on mRNA levels seen throughout the cell cycle, we concluded that antisense DNA caused specific hybrid arrest of translation. Our data suggest that prothymosin alpha is required for cell division. However, there is no evidence that prothymosin alpha directly regulates mitosis.