Parasporin (PS) is a collection of genealogically heterogeneous Cry proteins synthesized in Bacillus thuringiensis. A prominent feature commonly associated with PS proteins is the strong cytocidal activity preferential for human cancer cells of various origins. The proteins exhibit cytocidal activities only when digested with proteases. Currently, this protein group is classified into four families: PS1, PS2, PS3 and PS4. Marked differences are evident in cytotoxicity spectra and activity levels between the four PS families. Neither hemolytic activity nor insect toxicity is associated with PS proteins. One of the most striking aspects in the events induced by PS1Aa1 is the early and rapid increase of the intracellular Ca2+ concentration, with no change in plasma membrane permeability. There is strong evidence that PS1Aa1 kills cancer cells through apoptosis. Unlike PS1Aa1, PS2Aa1 increases plasma membrane permeability of cancer cells. The initial step in cytocidal action of PS2Aa1 is the specific binding of this cytotoxin to a putative receptor located in the lipid rafts, followed by its oligomerization and pore formation in plasma membrane.