Rabbit leukocytes activated in vitro by conconavalin A (Con A) or lipopolysaccharide (LPS) produce and release to the medium a neovascular attracting factor (NAF). In attempts to elucidate the possible role of some mediators of immune reactions, cyclic nucleotides, prostaglandins, synthetic chemoattractants, and growth factors were extensivly studied. Prostaglandin E1 was the most consistent and the strongest stimulator of new blood vessel proliferation. PGE2 and PGF2 alpha demonstrated a less stimulatory effect while PGF 1 alpha, PGD2, and PGA1 had no stimulatory potential. The cyclic nucleotides and the synthetic chemoattractants had no NAF potentials. Epidermal growth factor (EGF) demonstrated a moderate neovasculogenic activity, fibroblast growth factor (FGF) was less stimulatory than EFG, while nerve growth factor (NGF) had no stimulatory potential. Due to the outstanding and consistent neovasculogenic effect of PGE, a basic mechanism of neovascularization in which the prostaglandins may fulfill the role of regulators of blood vessel proliferation is suggested.