We have studied the role of 2 exogenous cytokines, the TGF beta 1 and the VEGF, on the in vitro angiogenesis process. Endothelial cells were plated on fibrin matrices either with 2% or with 10% human serum in 199 medium. Forty-eight hours later, with 10% human serum, the capillary-like network index (the percentage of the culture area covered by the capillary network) was about 50%. Under these culture conditions, when TGF-beta 1 or VEGF were added, the capillary-like network index augmented and was nearing 75%. When the index is high, using confocal microscopy, we show that hollow capillaries are formed. Moreover, the addition of VEGF increased the kinetics of the capillary-like network formation. With 2% human serum, 48 h after seeding the capillary-like network index was about 75%. In this case, the addition of TGF-beta 1 decreased the network index, whereas the addition of VEGF increased the kinetics of its formation. These in vitro angiogenesis experiments show that the serological factors underline the 2 antagonist effects of TGF-beta 1, but have no detectable effects on the activator effects on the VEGF.