Immunofluorescent studies were conducted for leukocyte-reactive antinuclear antibody (LR-ANA) with sera from 125 patients with ankylosing spondylitis (as), 124 with rheumatoid arthritis (RA) 74 with miscellaneous immune disorders (MID), 34 with acute inflammatory disorders (AID) and 122 non-immune controls. Positive reactions occurred with 60% of AS patients, 47% of RA, 40% of MID, 12% of AID, and 9% of non-immune controls. LR-ANA in AS sera invariably showed a homogeneous pattern of immunofluorescent staining with human granulocytes, occasionally reacted with human lymphocytes but did not react with other human and non-human substrates. Studies of 61 members of seven families with 18 cases of AS revealed a frequency of 38% LR-ANA, 30% AS and 55% HLA-B27, but no correlations were found among these parameters. These studies provide evidence of altered humoral immunity to human nucleic acids in the majority of patients with AS.