1. The cellular uptake and the efficiency in sensitizing cells to photoinactivation were determined for hematoporphyrin (Hp) diphenyl ether, Hp dicyclohexyl ether and Hp dihexyl ether. 2. The phenyl diether was taken up by the cells to the same degree as was the clinically used porphyrin preparation photofrin II, while the dihexyl and notably the dicyclohexyl ether were taken up 3-4 times better. 3. Furthermore, the quantum yields for photoinactivation of cells were similar for the three diethers and twice as large as that for photofrin II. 4. Fluorescence- and absorption spectroscopy indicate that these findings are related to the fact that photofrin II is much more aggregated in the cells than are the three Hp diethers. 5. When cells loaded with the porphyrins are incubated with porphyrin-free medium containing serum a certain percentage of the cell-bound drug is removed: 14% for photofrin II, 28% for Hp diphenyl ether, 50% for Hp dicyclohexyl ether and 20% for Hp dihexyl ether. 6. With respect to cell uptake and retention of the dyes, the data did not show any uniform relationship to the polarity of the drugs, in contrast to what has been found earlier for Hp diethers of linear hydrocarbons.