ABSTRACT It has long been known that the adhesiveness of leucocytes and blood platelets can be changed from a state of non-adhesion to very strong adhesion to each other, and to many surfaces, in a very short time. The agents that produce such changes are prostaglandins, thromboxanes and leukotrienes (see for example, Hoover et al. 1984), agonists affecting production of these substances and also those that affect the diacyl-glycerol-inositol triphosphate pathways (see Nishizuka, 1984). Evidence has recently emerged (Dunham et al. 1983; Dunham & Weissman, 1986) showing that sponge cells can have their adhesion activated by agents that affect calcium relationships of cells through diacylglycérol—ITP pathways. Brief pulses of low levels of calcium ions activate adhesion of these cells, phorbol myristoyl acetate and a calcium ionophore act synergistically in aiding cell adhesion. The activating agents can be effective at nanomolar concentrations. It is of interest to ask whether similar activation phenomena are more widespread than suspected.