The study was designed to determine the proper conditions for suspension culture of Viola tricolor cells in toxicity studies of Pb at different concentrations (0, 200, 500, 1000, 2000 \mu M) and exposure times (24, 48, 72 h). By forming insoluble salts with ions from the medium, lead (II) nitrate added to the medium decreased the initial 5.7-5.8 pH of the medium, depending on the Pb salt concentration and light intensity. In alamarBlue assays, we found no dose- or time-dependent effect of Pb on cell viability when we did not adjust pH and did not standardize the illumination conditions to correct the effect of lead-salt-induced turbidity. When effective illumination was adjusted to correct for turbidity at the highest lead concentration and pH was adjusted to 5.7-5.8, cell viability decreased with the increase of Pb(NO_{3})_{2} concentration and with treatment time. These experiments demonstrate that the toxic action of lead on cells in suspension depends strongly on culture conditions, and not only on the metal concentration and duration of treatment.