A gram-negative bacterium or its cell wall-defective variants were isolated from lymph nodes of ten patients with cat-scratch disease. Cultured bacteria were morphologically identical to vegetative and wall-defective forms seen in human tissues. Three of seven patients with recent cat-scratch disease had fourfold or greater rises in antibody titer against the cultured bacteria; the remaining four patients had maximum titers of 1:32 to 1:128. Rabbit antiserum to cultured bacilli reacted in immunoperoxidase stains with vegetative and wall-defective cat-scratch disease bacilli in lymph node, skin, or conjunctiva and with vegetative or wall-defective bacteria isolated from ten patients. Vegetative bacteria produced lesions in the skin of an armadillo identical to early lesions in human skin. Vegetative bacteria were recovered from the lesions in the armadillo.