The IgA and IgG antibody response to plasmid-encoded outer membrane proteins was studied in 59 patients with yersinia-associated spondylarthropathy during 15 months of follow-up. Initially, all patients had specific IgA and IgG antibodies to the 36 and 46 kDa and 30% also to the 26 and 58 kDa released proteins, which correlated with the finding of virulent Yersinia bacilli in intestinal biopsies. IgA disappeared in 69% of untreated patients after nine months and persisted in 31% after one year. IgA disappeared within three to six months in 81% of the patients treated with antibiotics for four to six weeks and persisted in 6% after one year (p less than 0.002). IgG antibodies to the 36 and 46 kDa outer membrane proteins persisted in 80% of all patients. Disappearance of IgA was coupled with disappearance of yersinia from intestinal biopsies.