Humoral antibodies and cell‐mediated immunity in patients with acute ulcerative gingivitis were investigated using antigens fromOdontomyces(Actinomyces) viscosus, Fusobacterium fusiforme, Veillotiella alcalescens and Bacteroides melaninogenicus. There were no differences in serum antibody levels between patients and control subjects on the first day of attendance and up to one month after the disease started. Serum and salivary IgA estimations failed to show a significant difference between patients and controls. Cell‐mediated immunity measured by the lymphocyte transformation test, yielded significantly different results between patients and controls. An immune pathogenesis for acute ulcerative gingivitis has been postulated to account for the immunological findi