Sparse HUVECs are more responsive to HGF than confluent HUVECs. HUVECs isolated from the same cord and grown as confluent or sparse monolayers were harvested and replated at the same density. They were then treated overnight with increasing concentrations of HGF: white bars, 1 ng/mL; gray bars, 5 ng/mL; and black bars, 25 ng/mL. The mitogenic response was assessed by measuring [³H]thymidine incorporation and expressed as a percentage of stimulation, calculated as follows: 100% × (cpm_experiment – cpm_control)/cpm_control, where the cpm values are the means from triplicate cultures. The 100% point was the background [³H]thymidine incorporation (2 × 10⁴ cpm) in the absence of HGF. The responsiveness of HUVECs to HGF is dependent significantly on the state of confluence of the cells (2-way ANOVA; P < .0001). Data are representative of 3 independent experiments. Error bars represent SDs.