Photodynamic therapy (PDT) induces among numerous cell targets membrane damage and alteration in cancer cell adhesiveness, an important parameter in cancer metastasis. We have previously shown that hematoporphyrin derivative (HPD)-PDT decreases cancer cell adhesiveness to endothelial cells in vitro and that it reduces the metastatic potential of cells injected into rats. The present study analyzes the influence of PDT in vivo on the metastatic potential of cancers cells and in vitro on the expression of molecules involved in adhesion and in the metastatic process. Photofrin w and benzoporphyrin derivative monoacid ring A (BPD) have been evaluated on two colon cancer cell lines obtained from the same cancer [progressive (PROb) and regressive (REGb)] with different metastatic properties. Studies of BPD and Photofrin w toxicity and phototoxicity are performed by colorimetric MTT assay on PROb and REGb cells to determine the PDT doses inducing around 25% cell death. Flow cytometry is then used to determine adhesion-molecule expression at the cell surface. ICAM-I, MHC-I, CD44V6 and its lectins (a `Ht1.3, PNA, SNA and UEA) are studied using cells treated either with BPD (50 ng/ml, 457 nm light, 10 J/cm 2 ) or Photofrin w (0.5 mg/ml, 514 nm light, 25 J/cm 2 ). Changes of metastatic patterns of PROb cells have been assessed by the subcutaneous injection of non-lethally treated BPD or Photofrin w cells and counting lung metastases. First, we confirm the metastatic potential reduction induced by PDT with respectively a 71 or 96% decrease of the mean number of metastases (as compared with controls) for PROb cells treated with 50 ng/ml BPD and 10 or 20 J/cm 2 irradiation. Concerning Photofrin w -PDT-treated cells, we find respectively a 90 or 97% decrease (as compared with controls) of the mean number of metastases for PROb cells treated with 0.5 mg/ml Photofrin w and 25 or 50 J/ cm 2 irradiation. Then, we observe that CD44V6, its lectins (aHt1.3, PNA, SNA) and MHC-I are significantly decreased (compared with the òther molecules tested) in PROb and REGb cells after both BPD and Photofrin w PDT treatment. These modifications in adhesion-molecule expression, particularly of CD44V6, can thus account only for part of the decrease in the metastatic potential of PDT-treated cancer cells. Changes in adhesion-molecule expression induced by PDT are only transient, implying that the rate of metastatic reduction is probably not linked simply to these changes.