In recent years porphyrins and related materials have been tested as antitumor agents. A technique was devised to obtain dose-response curves for the sensitizer fraction that resists one day of elution by tissue culture medium--the retained fraction. We found a steep "threshold" dose response relationship that helps to explain tumor destruction without damage to normal tissues. The family of dose-response curves produced by a wide range of light exposures suggests that chlorins and porphyrins do not act by identical mechanisms. Moreover, they suggest that chlorins will prove superior in practical use.