Tissue-cultured seedlings of mountain laurel (Kalmia latifolia L.) were treated with strain AOK-30 of Streptomyces padanus in flasks. This treatment accelerated pigmentation of the seedlings. The untreated green seedlings wilted in room conditions within 1 hr of removal from the flasks while the treated reddish seedlings were still turgid. Fresh weight of leaf pieces decreased more quickly in the untreated than in the treated specimens, suggesting that the AOK-30 treatment somehow rendered the seedlings drought-tolerant. Protoplasts were released from untreated green seedlings with cellulase and macerozyme, while xylanase was required in addition to cellulase for protoplast preparation from the treated reddish seedlings, suggesting that composition and structure of the cell walls of seedlings may have been modified by the AOK-30 treatment. Osmotic pressure was higher in protoplasts from the treated than the untreated seedlings. Elevation of osmotic pressure in cells and cell wall modification may partially account for the enhanced drought-tolerance of the AOK-30-treated seedlings.