We present results of preliminary studies on 5-aminolevulinic acid (5-ALA)-photodynamic therapy (PDT). In order to assess the effectiveness of 5-ALA-PDT we have used BALB/c mice transplanted subcutaneously with mouse colon adenocarcinoma C51. 5-ALA in the dose of 50 mg/kg was given intraperitoneally and 5 h later tumors were exposed to light at total doses from 50 to 75 J/cm2, wavelength 630+/-20 nm and light intensity 200 mW/cm2. Several time points following 5-ALA injections have been used to measure protoporphyrin IX (PpIX) concentration in different tissues from unirradiated mice. PDT effects were evaluated with regard to survival time, tumor response and necrosis depth. The main finding in our tumor model was that the optimum tumor to skin ratio of PpIX occurs within 5 h after sensitizer injection. 5-ALA-PDT resulted in prolongation of survival time of treated mice as compared to control animals and, in some cases, in complete response of tumors. PDT also caused increase in tumor necrosis, while no skin photodamage was observed.