To study the photosensitizing properties of bacteriochlorin a (BCA) in a (lipo)protein-rich environment, the photosensitizing efficacy was tested by clonogenic survival of Chinese hamster ovary and T24 (human bladder carcinoma) cells. Confluent cell layers were incubated with 2.5 tzg m1-1 BCA in cell culture medium for 1, 4, 6, 18 and 24 h. Upon illumination with red light it was found that BCA was not effective as a photosensitizer in this medium. Extraction methods showed that this lack of photosensitization could not be explained by the inability of the dye to enter the cells in the presence of cell culture medium. The presence of cell culture medium did not change the spectral properties of BCA to an appreciable extent. Standard KBr density gradient ultracentrifugation showed that in the presence of cell culture medium approximately 20% of the BCA was sedimented with low density lipoprotein (LDL) and 60% with high density lipoprotein (HDL). Incubating T24 cells 18 h before the clonogenic cell survival assay in serum-deficient medium restored the photosensitizing properties of BCA. It is proposed that in a protein-rich (in vivo) environment BCA associates with lipoproteins and can be taken up by malignant neoplasms via the LDL pathway.