The human platelet and erythrocyte differ quite dramatically in relation to the arachidonic acid cascade. The platelet synthesizes its own characteristic products, while the erythrocyte lacks cyclo-oxygenase activity but possesses other metabolic enzymes. In these and other cell types the metabolites are potentially determined by the cascade enzymes, the availability of cofactors, the presence of specific activators and inhibitors, and the selective binding or transport of intermediates. Endogenously synthesized metabolites may have intracellular actions or may be released and exert their effects extracellularly. Study of the cellular mechanisms mediating these effects, like study of the fast-acting hormones, has focused primarily on the cyclic nucleotides and calcium. Considering the diverse activities of several metabolites in the platelet however, these mechanisms seem to need reevaluation or refining. The released cascade metabolites may also act as intercellular signals over a short range. The range depends on their chemical stability in the absence of protective carriers and their selective uptake and metabolism by surrounding cells. Additionally, the effects will reflect the selective interaction of responsive cells with the spectrum of metabolites released. Answering these questions of complex intercellular interactions requires the identification and classification of characteristic responses and the metabolic profile of individual cell types in each tissue. Consequently, this type of analysis may best be done with isolated cells, such as the platelet and erythrocyte.