1. Heat does not sensitize paramecia to ultraviolet light but ultraviolet light sensitizes them to heat. Paramecia of two species (Paramecium caudatum and P. multimicronucleate) are much more readily killed by heat at 42.3° C. if they are first exposed to ultraviolet light. 2. From studies on paramecia irradiated with a given dosage at various wave lengths before being killed by heat, an action spectrum of the compound in the protoplasm being sensitized to heat can be determined. Proteins with absorption similar to that of pseudoglobulin are suggested by these experiments. 3. The effect upon living things differs from that on pure protein systems in that paramecia are not rendered more sensitive to temperatures below the lethal temperature whereas proteins are. 4. Almost complete recovery from ultraviolet light as judged by heat sensitivity occurs within 4 to 5 days. 5. By a study of the rate of recovery from doses at different wave lengths evidence suggesting effects on nucleic acid is obtained. 6. The possible significance of the data and the action spectrum is discussed.