The effect of polycationic polypeptides (polylysine, polyarginine and polyhistidine) on the invasion of mammalian cells and plant protoplasts by Toxoplasma gondii was studied. In JM cells, a human lymphoblastoid cell line with T cell characteristics, all polycationic polypeptides used increased the invasion rate in a concentration-dependent manner. This effect and the morphological changes revealed by electron microscopy resembled the action of the penetration-enhancing factor previously described by E. Lycke and co-workers. Plant protoplasts of Catharanthus roseus, which are resistant to T. gondii invasion, showed the same morphological changes in the presence of polycationic polypeptides as observed for JM cells, but were not invaded.