A 27-year-old female lowland gorilla developed an asymmetric oligoarthritis 3 months post-partum. There was no evidence of an antecedent gastrointestinal or genitourinary infection. Serum was negative for rheumatoid factor and antinuclear antibody. Synovial fluid revealed 2000 white blood cells with negative cultures and polarized microscopy. Studies on synoviocytes were the following: (1) FACS analysis revealed surface expression of a B27-like epitope of the cells. (2) Analysis of intracellular clearance kinetics of arthritogenic organisms showed peak intracellular colony-forming units at 48 hours after bacterial invasion, and clearance by 13 days post-invasion. (3) Interferon-y (0.1-10.0 ng/ml)accelerated intracellular microbicidal pathways in a dose-dependent fashion. These findings closely parallel those seen in human synoviocytes of patients with spondyloarthropathy. Primate and human seronegative arthritis share clinical and immunologic features, as well as aspects of host:pathogen defense mechanisms. The interplay of genetic and microbial factors underlying this arthritis appears to be conserved across these species boundaries.