The capacity of mononuclear blood cells to produce interleukin-1 alpha (IL-1 alpha) after stimulation with Porphyromonas gingivalis in cell culture was studied. The results obtained with cells from periodontitis patients were compared with those from a control population. The concentration of IL-1 alpha in serum and saliva was also determined and compared with the concentration in mononuclear blood cell cultures. No significant relationship was found between the incidence of periodontitis or severity of the lesions and IL-1 alpha production in the presence of P. gingivalis. Nevertheless, 11 of 30 periodontitis patients, showed levels > 30 pg/ml of IL-1 alpha in mononuclear blood cell cultures stimulated by P. gingivalis, whereas only three healthy control showed these titers of IL-1 alpha.