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The signal transduction mechanisms involved in complement fragment C5a-induced recruitment of actin to the cytoskeleton have been investigated using U-937 cells differentiated by exposure to dibutyryl cyclic AMP. Two parameters of cytoskeletal activation were compared: F-actin formation and nucleation of polymerization of pyrenyl-actin in whole cell lysates. The dose dependency of these responses to C5a was clearly different to that observed for [³H]inositol phosphate formation and also markedly different from that observed for the production of reactive oxygen intermediates (ROI). Further evidence to dissociate inositol lipid hydrolysis from these cytoskeletal responses was obtained by treating cells with neomycin, phorbol myristate acetate and pertussis toxin and by modulating the levels of intracellular Ca²⁺ using quin 2. Inhibition of [³H]inositol phosphate and ROI production was not correlated with effects on actin recruitment or nucleation. In addition, these agents had differing effects on F-actin formation and nucleation activity. The results show that the production of inositol phosphates is not required for stimulating either F-actin formation or nucleation activity and also that ligand-induced polymerization of actin depends primarily upon an increase in the availability of G-actin rather than nucleation sites. These cytoskeletal responses are apparently controlled by different signalling pathways which diverge at an early stage.