SUMMARY.— The mechanism by which the Koebner reaction could be provoked was studied in 6 patients with active psoriasis. It was concluded that purely epidermal or purely dermal damage was ineffective but that damage of the basement membrane was not essential. The factors determining a successful reaction are not clear. Injury of both epidermis and dermis appears to be essential but it is perhaps the degree of tissue damage which is important. Copyright © 1969, Wiley Blackwell. All rights reserved