Collagen-induced arthritis has been widely used as an animal model of rheumatoid arthritis. We have used this model with a view to determining potential therapeutic targets for the treatment of human disease. To do this we have attempted to modulate the progression of established arthritis over a 10-day time period following the first appearance of disease, by i.p. injection of one of three different MoAbs. These consist of a rat IgG2a specific for the CD5 antigen expressed on all T cells and a subpopulation of B cells, a mouse IgG2b recognizing the CD72 antigen, and a rat IgM specific for the B220 molecule, CD72 and B220 both being expressed on all B cells. None of the three MoAbs had depleting activity in vivo. The progression of arthritis was monitored both clinically, and histologically. The effects of treatment with anti-CD5 and anti-CD72 antibodies were compared with control antibodies of the same species class and subclass. In the case of anti-B220 antibodies, the effects of treatment were compared with administration of PBS. Of these MoAbs, only treatment with anti-CD5 resulted in disease amelioration with significant decrease in disease severity in 60% of the animals. These changes became apparent 6 days after initiation of treatment. There were no significant differences in serum levels of IgG antibodies to native bovine collagen type II between the groups of treated and control mice. Possible mechanisms underlying the modification of disease expression following treatment with anti-CD5 MoAb are discussed.