Macrophage activation, broadly defined as the acquisition of competence to execute a complex function, continues to intrigue biologists, because of its fundamental importance to the host and the fascinating scientific challenges it poses. A recent meeting* emphasized the considerable progress made in this area since the topic was last considered in such a workshop. The workshop focused sharply on one aspect of the field: the molecular basis of macrophage activation, and covered the multiple, specific signals that induce and suppress activation, how these signals are transduced and thus affect macrophage behaviour and how induced function, in turn, is effected.