Particles carrying C3 in a random distribution (Etan-C3) bound to C3 receptor (Raji+) cells independent of temperature and irrespective of whether the Raji cells were fixed with glutardialdehyde. In contrast, the reactivity of EAC43b, having grouped C3b in clusters, was dependent on temperature. The interaction with Raji cells was inhibited if the latter were treated with a fixative. The reaction of both Etan-C3 and EAC43b was a function of the concentration of C3 and C3b, respectively. The conclusion is drawn that for the interaction between C3/C3b-carrying particles and C receptor cells the receptors and the ligands have to be grouped in a similar typographic distribution, irrespective if this arrangement is provided already from the beginning or if it is obtained only by lateral movement of one or both reaction partners.