The difference in extracellular pH in malignant as compared to normal healthy tissues has been proposed to contribute to selective uptake of photosensitizers in tumors. Hematoporphyrin IX (HpIX), disulfonated meso-tetraphenylporphine (TPPS 2 a ), meso-tetra(3-hydroxyphenyl)porphine (mTHPP) and meso-tetra(3-hydroxyphenyl)chlorin (mTHPC) were chosen to examine the pH dependence of their cellular drug uptake. The study was performed in the pH range 6.5-8.0 and showed that significantly higher amounts of the drug are taken up by T-47D cells at low pH values only in the case of HpIX. The pH value of the incubation medium did not influence the cellular uptake of mTHPP, mTHPC and TPPS 2 a significantly. The present work indicates that tumor selectivity of dyes, which get more lipophilic with decreasing pH value, may be related to the low extracellular pH value.