Photodynamic diagnosis and therapy (PDD & PDT) of cancer use generally photosensitizers for the discrimination of normal and cancer cells. In contrast we have studied the autofluorescence spectra of normal and cancer cells of rat using a tunable optical parametric oscillator (OPO) laser system as a light excitation source. WKA rat fibroblast (normal cell) and H-ras oncogene-transfected rat fibroblast ( cancer cell) were used in this study. The samples were mounted in a quartz cuvette. The absorption peaks were observed at about 205 nm for both normal and cancer cells. On the other hand, the specific emission peaks were observed at 450 nm and 455 nm for normal and cancer cells, respectively, whereas the autofluorescence intensity of cancer cells was 2.5 times larger than that of normal cells. These changes are attributed to changes in proteins in the cancer cell and to the increase of nuclei. This suggests the possibility that the autofluorescence of cancer cell can be used to diagnose a human malignant tumor, so that it is concluded that normal and cancer cells can be discriminated by means of autofluorescence analysis.