SENSITIZATION OF CELLS TO HEAT BY VISIBLE LIGHT IN PRESENCE OF PHOTODYNAMIC DYES.

1. Visible light of high intensity does not injure paramecia or sensitize them to heat. 2. If photodynamic dyes are added, paramecia are readily killed by visible light of high intensity and are sensitized to heat by sublethal dosages of light. 3. Cells so sensitized are killed when subjected to a sublethal exposure to heat. 4. If the light and heat are applied in the reverse order, namely, heat and then light, no ill effects are observed. 5. When the concentration of dye is reduced a larger light dosage is required. 6. Recovery from sensitization is slow, requiring about 4 days for a (3/4) lethal dosage. 7. Sublethal dosages of light in the presence of dyes do not affect the division rate even when (3/4) the lethal dosage has been used. 8. A possible explanation for the photodynamic sensitization to heat is discussed.