The photosensitizer hematoporphyrin (HP) was site specifically attached to a murine monoclonal antibody (MAb) fragment OC125F(ab?)(2) directed against ovarian cancer cells and to nonspecific rabbit immunoglobulin G. The photoimmunoconjugates were positively charged and were purified by column chromatography. The OC125F(ab?)(2) conjugate retained immunoreactivity with human ovarian cancer cells, and the binding was competed with unmodified MAb. Phototoxicity paralleled the cellular uptake with the OC125F(ab?)(2) conjugate and the light showing selective killing of target cells compared with nontarget cells. Nontargeted conjugates and free HP produced lower levels of phototoxicity and showed no selectivity.