A new quantitative assay for measuring angiogenesis in a s.c. located sponge implant in rats is described. Using this model, which detects neovascularization by measuring alterations in 133Xe clearance, it has been shown that the known angiogenic factors, transforming growth factor α and tumor necrosis factor α, cause maximum vascularization of the sponge to occur by Day 11 postimplantation compared with Days 15 to 17 in control animals. The monokine interleukin 1α is shown to be strongly angiogenic, suggesting that more than one macrophage-derived cytokine may be the active mediator in macrophage-induced angiogenesis. Extracellular matrix proteins appear to play a role in regulating the angiogenic response such that presoaking sponges in laminin (40 µg/ml) or fibrinogen (500 µg/ml) solutions induced a significant reduction in the time taken to achieve maximum 133Xe clearance values; no such enhancement of neovascularization was observed when sponges were presoaked in type IV collagen (100 µg/ml) solution. The assay described here, which is reproducible, objective, and quantitative, should be of considerable use in elucidating the molecular basis of angiogenesis regulation.