Basic fibroblast growth factor (bFGF), is a heparin-binding factor with potent angiogenic properties in vitro and in vivo. bFGF is involved in tumour growth, but it has also been shown to reduce infarct size in experimentally induced acute myocardial infarction. Platelets are also believed to have an important role in both tumour growth and myocardial infarction. We have studied bFGF binding to platelets by flow cytometry. Platelet activation by ADP induces bFGF binding to platelets. bFGF bound to activated platelets will result in a locally high concentration of bFGF in patients with myocardial infarctions and malignant tumours. Addition of recombinant bFGF to platelet rich plasma reduced the percentage of fibrinogen positive platelets. bFGF may thus have an inhibitory effect on platelet aggregation.