A series of compounds based on an asymmetrical protoporphyrin molecule have been examined. The paired groups of sensitizers differed in terms of the presence or absence of a permanent positive charge, in the alkyl side chain length and in having either a primary or secondary amine substituent. The effects of these variables on drug uptake, partition coefficient and photodynamic cell kill were tested. Drug uptake and partition coefficient were shown to be correlated. Differences in gross uptake were found within paired groups of sensitizers although cell-associated uptake alone did not correlate with clonogenic cell survival. Of the compounds tested it was the sensitizers with alkyl side chains, rather than the permanently positively charged compounds, which resulted in the greatest degree of clonogenic cell kill.