Thirty percent of patients undergoing percutaneous transluminal coronary angioplasty develop recurrent disease within a year. This is usually due to the rapid accumulation of intimal smooth muscle cells and extracellular matrix, which causes luminal narrowing, and is probably orchestrated by several mitogenic and chemotactic factors, of which platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF) appear to be particularly important. We have investigated the effects of administering a combination of neutralizing antibodies directed against PDGF-BB and bFGF on neo-intima development following balloon catheter injury in the rat carotid artery. Purified sheep anti-PDGF-BB and anti-bFGF immunoglobulins (IgGs) were administered singly and in combination prior to mechanical injury and daily until sacrifice, 8 days later. Plasma titres of exogenous anti-PDGF-BB and anti-bFGF were maintained at levels 10-20-fold higher than those required to neutralise the mitogenic and chemotactic effects of 20 ng/ml of PDGF-BB, or 10 ng/ml bFGF in vitro. Used singly, anti-PDGF IgG treatment was associated with a 47% reduction in intimal thickness and a 59% reduction in intimal:medial area ratio; anti-bFGF IgG administration caused a 53% reduction in intimal thickness, and a 50% reduction in intimal:medial area ratio. Treatment with a combination of these antibodies resulted in a 83.8% reduction in intimal thickness (P < 0.05), and a 91% reduction in intimal:medial area ratio (P < 0.01). The latter treatment was also associated with a significantly higher intimal cell density (14.2 +/- 1.6 x 10(3) nuclei/mm2) compared to animals receiving non-immune IgG (7.8 +/- 0.8 x 10(3) nuclei/mm2; P < 0.025), although intimal and medial cell proliferation indices were not significantly different between the groups (P > 0.05). Our results suggest that in this particular model, PDGF-BB and bFGF are the major factors controlling neointimal hyperplasia, and that these growth factors are operating principally via an effect on smooth muscle cell migration and extracellular matrix protein accumulation.