Russian thistle (Salsola kali), the most common plant referred to as "tumbleweed" in the western United States, can cause a dermatitis in persons who come into direct contract with it. Tests were conducted to determine the mechanism of this dermatitis. Mechanical contact with plant branches, as well as scratch, patch, and photopatch tests with Russian thistle extract and scratch tests with 1.5% potassium nitrate, a plant constituent, were employed. These tests, along with transparent adhesive tape preparations and a skin biopsy, showed that in nonsensitized persons, dermatitis was due only to mechanical irritation of plant floral bracts. In sensitized individuals, Russian thistle floral bracts pierced the skin and stimulated an urticarial reaction.