The influence of both age and sex on antigen-induced arthritis in C57Bl/10 mice was studied. Methylated bovine serum albumin was used to induce arthritis in young adult (3 months) and old (18 months) male and female mice. Arthritis became chronic and led to severe joint damage more often in 18-month-old female mice, compared with both young adult female mice and with male mice of both ages. T cell immunity and levels of antibodies against methylated bovine serum albumin were comparable in all groups. Antigen retention in the joint was greater in old compared with young adult mice of both sexes, and could therefore not entirely explain the increased prevalence of persistent arthritis in old females. Our data suggest that female hormones, in combination with age, are important factors in the chronicity and destructive character of this type of arthritis.