Synergism between specific antibodies and cytotoxic drugs in killing tumor cells is an established phenomenon. When cells grown in a suspension culture are treated with antibody, a subsequent increase in susceptibility to cytosine arabinoside has been shown to be due to the induction of cell-cycle synchrony. This observation may lead to a novel way of investigating cell-cycle-dependent processes and also an aspect of tumor-antibody interactions that is not generally recognized.