THE movement of mammalian cells can be inhibited by agents which mimic or produce increases in the intracellular levels of cyclic 3′,5′-adenosine monophosphate (cyclic AMP)1,2. Goldberg and coworkers3,4 have proposed that in a number of cell types cyclic 3′,5′-guanosine monophosphate (cyclic GMP) promotes cellular events that are antagonistic to those believed to be mediated by cyclic AMP. One method of testing this hypothesis would be to examine the effects on leukocyte chemotaxis of agents that should stimulate an increase in cellular cyclic GMP concentration. Both phorbol myristate acetate (PMA)5 and acetylcholine (Ach)3 seem to produce such an effect on more than one cell type and these compounds were therefore used to investigate the possibility that cyclic GMP may play a role in modulating cell movement.