Preparation of the tumor-localizing preparation called hematoporphyrin derivative involves a two-step reaction wherein hematoporphyrin is acetylated and the reaction product subjected to alkaline hydrolysis. We have proposed that the tumor-localizing fraction of this product is composed of hematoporphyrin units joined by ester linkages. Using an analogous synthetic procedure, we prepared some new sensitizers in which hematoporphyrin is esterified to chlorins (reduced porphyrins). Because of the differences in absorption spectra, the porphyrin-chlorin esters are 5-fold more potent photosensitizers than are the porphyrin-porphyrin esters, with light of wavelength greater than 600 nm.