A collagen-derived octapeptide KPGEPGPK which specifically inhibits the activation of platelets by collagen has been tested for its ability to affect the collagen-induced phosphoinositide breakdown and protein phosphorylations. Collagen produced a transient decrease followed by a rapid resynthesis of [32P]-phosphatidyl 4-5 bisphosphate (PIP2) and 4-mono phosphate (PIP). Octapeptide, at a concentration preventing aggregation but allowing shape change, did not impair the phosphoinositide breakdown, whereas the P43 phosphorylation was strongly inhibited. Higher concentrations of peptide which did not permit any shape change were needed to hinder the PIP2 and PIP decrease. Therefore, the octapeptide appears to affect early events of the collagen-induced platelet activation involving the P43 phosphorylation, independently of its effect on the receptor-stimulated phosphoinositide hydrolysis.