The photobiological activity of mesotetra (4N-methylpyridyl) porphine (T4MPyP) has been studied in a mammalian cell line in vitro. The uptake of T4MPyP to HeLa cells was examined using fluorescence microscopy. Under violet-blue exciting light, a nucleus-selective red fluorescence was detected. The intensity of fluorescence depended on drug concentration and incubation time, with a maximum relative emission at a concentration value of 10(-5) M for 20 min. The photosensitivity of HeLa cells incubated with T4MPyP under previous conditions and subsequently exposed to variable doses of visible light has been examined. Cell toxicity was evaluated as the surviving fraction, using the trypan blue staining test. The extent of cellular damage was found to be dependent on irradiation time, with a lethal damage after 8 min of light exposure. Cellular DNA-photodamage was studied under sublethal conditions (2 min of irradiation) by the use of the micronucleus test. The number of micronuclei, in these conditions, increased with regard to baseline levels.