Abstract Poliovirus, as usually found in fluids of infected cultures, cannot be inactivated photodynamically in the presence of heterotricyclic dyes. However, if the mature virus is freed of extraneous organic material, dyes can attach to the virus and render it light-sensitive. Optimal conditions for photosensitivity of poliovirus were found to be pH 8.0 in phosphate buffer in 10 −4 M dye (neutral red, toluidine blue, or proflavine). Organic buffers (Tris and glycine) prevent photosensitization of poliovirus. When the pH is lowered, the photosensitive virus-dye complex appears to dissociate and the free virus again becomes light-resistant. A cation exchange column adsorbs most of the virus-dye complex. That part of the virus which dissociates from the dye on the column is no longer photosensitive. The attachment of dye to vaccinia virus is also reversible, and virus resistant to light can be obtained by chromatographic adsorption of dye from the virus-dye complex.