The effects of different factors (temperature, light, enzymic activation) on the ability of selected mutagens and carcinogens to induce hereditary bleaching of Euglena gracilis were investigated. In the resting medium, the elevation of incubation temperature from 25 degrees C to 37 degrees C increased significantly the effect of all compounds tested on the frequency of bleached mutants of E. gracilis. The effect of light is not so unambiguous. While nitrosoguanidine (NG) exhibited practically the same bleaching activity both in the light and dark, the mutagenic effect of sodium azide (SA), nitrovin (NV), nitrosoethylurea (NEU), and benzo(a)pyrene (BP) was decreased in light. On the other hand, the light increased the bleaching activity of 5-nitro-2-furylacrylic acid (NFAA) significantly. The activation mixture S9 increased bleaching effect of NFAA and BP, whereas other mutagens were partially (NG and SA) or completely (NV and NEU) inactivated.