Moderately high molecular weight polycations stimulate arachidonic acid release with concomitant synthesis and release of prostaglandins in cultured 3T3 mouse fibroblasts. We have examined a series of synthetic polycations for prostaglandin synthesis-inducing activity as an approach to defining the structural features required for activity. Extensive (>80%) acetylation of poly(vinylamine) was tolerated without loss of activity, indicating that a uniform high density of charges is not required. However, complete acetylation of poly(vinylamine) abolished activity, indicating that some positive charges are required for activity. Full activity was observed for charge densities in the range of one per two to one per six atoms of polymer backbone. Branched and linear polycations activated equally well. Location of the charge with respect to the polymer backbone did not affect activity in polymers bearing charges located up to seven atoms away from the backbone. Polycations lacking primary or secondary amino groups exhibited full activity, indicating that Schiff base formation is not required for activity. These results are consistent with a model in which activation involves electrostatic interactions with discrete anionic sites on the target cell.