The effect of exogenously supplied NAA on Zantedeschia \'Ruby\' adventitious root formation and shoot growth in vitro was studied by applying concentrations of 0.01-10.0 mg l^-1. Transfer of plantlets from MS nutrient medium containing 0.3 mg l^-1 of BAP to the medium containing various NAA concentrations stimulated growth of shoots only at NAA concentrations of 0.01-0.1 mg l^-1. At concentration 5.0-10.0 mg l^-1 of NAA the percentage of explants with roots on the MS medium decreased from 100% to 60.7-77.4%, respectively. Shoot length was best affected by NAA concentration of 0.1 mg l^-1. NAA concentration of 0.01 mg l^-1 was the optimum one for the best development of plants with branching roots. The results indicated that NAA concentration is an important factor for root formation.