Experiments were carried out on cancerous HeLa cells and blood serum using a double integrating sphere and a He-Ne laser to investigate the optical properties and cellular effects due to photodynamic therapy (PDT). In the first experiment, HeLa cells were exposed to Photofrin at concentrations of 0, 10, 20, 30, 50 and 112.4 µg/ml at an irradiance of 0.2 W/cm2 using diode laser light. Using a confocal microscope, cell debris and morphological changes in HeLa cells were recorded at different Photofrin concentrations. The results showed cell debris in HeLa cells at the highest concentration of Photofrin. In a second experiment, photobleaching was observed in HeLa cells in the presence of various concentrations of 5-aminolaevulinic acid ranging from 0–50 µg/ml. There was progressive degradation of the 635 nm peak during continuous laser irradiation at an irradiance of 0.2 W/cm2. We conclude that cells demonstrating high initial fluorescence undergo bleaching at a faster rate than those with lower fluorescence. Finally in a third experiment, cancerous and noncancerous blood serum was irradiated at an irradiance of 0.1 W/cm2 using a He-Ne laser in conjunction with a double integrating sphere system. Forward and back scattering of normal and malignant serum showed an exponential decrease in fluorescence amplitude. The results indicate that there is notable amplitude difference between malignant and normal blood serum with malignant blood serum showing decreased scattering. These results have important implications for photodiagnosis and photodynamic therapy.