Summary The effects of photodynamic treatment (PDT) on venules include vascular leakage accompanied by oedema formation, vasoconstriction and blood flow stasis. The goal of this study was to gain insight into the mechanism underlying these vascular events by studying one of the earliest observations after PDT, granulocyte adhesion, in an in vitro model. For this purpose human umbilical vein endothelial cells (HUVECs) preincubated with Photofrin II (PII) were illuminated with red light and incubated with neutrophils. PDT led to a dramatic change in the morphology of the endothelial cells. Clearly, neutrophils adhered to the subendothelial matrix and their adherence coincided with an increase in the percentage of exposed subendothelial matrix by the gradual contraction of endothelial cells. Furthermore, the increase in adherence was dependent on drug dose, illumination time and the time delay after PDT. The neutrophil adherence could be inhibited by anti-,B2-integrin antibodies, which suggests that the aL-, oM-or OCx-fl2 receptors of the neutrophil mediated this phenomenon. At 4°C or by preincubation of the neutrophils with staurosporin, their adherence to the subendothelial matrix exposed by PDT of endothelial cells could be prevented. Apparently, activation of the fl2-integrin receptor by interaction with the subendothelial matrix is necessary for the increased binding of neutrophils. Taken together, these in vitro findings suggest that the PDT-induced contraction of the endothelial cells permits neutrophil adherence to the subendothelial matrix. It is conceivable that a similar mechanism contributes to the initial adherence of granulocytes to the vessel wall as observed after PDT in vivo.