Plasminogen activator inhibitor type-1 (PAI-1), the primary regulator of plasminogen activator - urokinase (uPA) plays a crucial role in the cell adhesion and migration and in angiogenesis. We had previously demonstrated that PAI-1 - endothelial cell interplay is critical for the formation of new blood vessels and the process is mostly conducted via uPA- anti-proteinase interaction. In the present study we wished to further examine the role of PAI-1 in the sprout formation, representing the first step of new capillary vessels development by evaluating the effect of PAI-1 on the sprout area. We addressed the issue by assessing the influence of cysteine-mutated PAI-1 proteins characterized by a prolonged half-life time (hD beta T - T(1/2)= 63.59 h and beta T-T(1/2)= 6931.47 h), and therefore more stable anti-uPA activity, on the appearance of newly formed sprouts. We found that both CysPAI-1 proteins significantly diminished the mean sprout area in a concentration-dependent fashion. The inhibitory effect present in the two examined endothelial cells systems of different origin and functional characteristics - human umbilical vein endothelial cells (HUVEC) and human lung microvascular endothelial cells (HLMVEC) cultures - was noticeably greater for HLMVEC -high urokinase-producers. Moreover, the inhibition rate was significantly greater for the beta T mutant than that for the hD beta T PAI-1 mutant in all examined doses (P<0.002), proving a key role of anti-proteinase activity for this effect. We concluded that, apart from the total sprout length, as it had repeatedly been demonstrated before, also affects the sprout area in the in vitro sprout formation angiogenesis assay. This effect was achieved mainly via PAI-1's antiproteinase activity.