A comparative study of the uptake and photoinactivation of HeLa cells treated with methylene blue (MB) and toluidine blue (TB) has been performed. Cell toxicity induced by different concentrations of either MB or TB showed that 10(-5) M was the concentration at which dark damage was not observed, while an elevated photoinactivation could be detected with both thiazines. The uptake studies showed that the penetration kinetics of 10(-5) M MB into HeLa cells is faster than that of TB, used at the same concentration, reaching saturation (approximately 1.8 micrograms/10(6) cells) after 6 or 12 h of incubation, respectively. For both sensitizers, the survival of HeLa cells was dependent on the incubation time, as well as the light dose, for a given concentration (10(-5) M). Moreover, cell photoinactivation produced by MB was higher than that produced by TB. The differences between MB and TB obtained under our experimental conditions are discussed.