Downmodulation of adjuvant arthritis (AA) by in vivo tumour necrosis factor (TNF)α treatment of Lewis (LEW) rats. LEW rats were injected intraperitoneally daily either with 1 ml of 105 U/ml TNFα (n = 4; experimental group; ■) or with 1 ml PBS (n = 8; controls; □) for 3 days before the day of Mycobacterium tuberculosis H37Ra (Mtb) injection, and then continued daily for 7 days, including the day of Mtb injection, to a total of 10 injections. Thereafter, all rats were observed for signs of AA, and the severity of arthritis was graded as described in Materials and methods. The difference in the severity of arthritis during the course of AA in the two groups of rats was statistically significant from day 16 through day 25 (*p < 0.05; **p < 0.025). The difference between the two rat groups was also significant (p < 0.05) when analysed by Wilcoxon rank sum test. Similar results were obtained in repeat experiments. Also shown in the figure is a representative designation of different phases of the disease in the course of AA in the form of days post-Mtb immunisation as follows: Inc = incubation, days 1 to 7; Ons = onset, days 8 to 10; Pk = peak, days 15 to 18; and Rec = recovery, days 23 to 30.