Stock plants of `Shepody' and `Yukon Gold' potato (Solarium tuberosum L.) were grown in a greenhouse and irrigated with city water. Contamination rate of stem explant tissue cultures excised from these stock plants was 50% to 100%. A comparison of the microorganisms isolated from the contaminated cultures and from 0.22-μm filter disks through which 20 liters of city water had passed revealed the presence of similar bacterial floras. Five genera of bacteria (Listerium spp., Corynebacterium spp., Enterobacter spp., Pasteurella spp., and Actinobacillus spp.) were isolated from contaminated cultures and cultured filter disks. Watering greenhouse-grown stock plants with filtered city water decreased contamination of stem explant cultures 30% to 50%. Installing an ultraviolet light water-sterilizing unit at the greenhouse inlet point effectively reduced contamination.