The plasma distribution and biodistribution of benzoporphyrin derivative were examined. Two analogs of benzoporphyrin derivative were mixed with human plasma in vitro and recovered in the lipoprotein fractions upon separation by chromatography or ultracentrifugation. The majority of both analogs was recovered with high density lipoprotein. The effect of prebinding benzoporphyrin derivative to lipoproteins on the biodistribution of the drug in vivo was studied in tumor bearing DBA/2J mice. At 3, 8 and 24 h post-injection, tumor and tissue samples were excised and analyzed for benzoporphyrin derivative content. Precomplexing benzoporphyrin derivative with low density lipoprotein or high density lipoprotein led to significantly (P less than 0.05) greater tumor accumulation than in aqueous solution.