This study investigates immunogenicity and in vivo effects on T-cells of long-term CD4 monoclonal antibody treatment of patients with rheumatoid arthritis. Patients were treated with several dosage regimens of a chimeric CD4 monoclonal antibody entitled cM-T412 over the course of 1 year. The circulating CD4+ T-cell count sharply decreased after the first cM-T412 injection and slowly recovered after the last injection. Within the CD4+ subset there was a selective depletion of CD45RA+ T cells, HLA-DR+, and CD25+ cells, providing evidence that activated/memory CD4+ cells resist the effect of CD4 monoclonal antibodies. Studies on cytokine production by peripheral blood mononuclear cells cultures in vitro revealed no differential effect on the production of interleukin-4 compared to interferon-gamma, indicating that a shift from a Th1 to a Th2 lymphokine production profile was not achieved. Human anti-monoclonal antibodies (HAMA) were induced in a minority of the patients predominantly after the first treatment course. All the sera containing HAMA specifically inhibited the binding of cM-T412 to T-cells. However, HAMA formation does not interfere with the biological effect of repeated cM-T412 administration since the degree of CD4 depletion following repeated administration of cM-T412 to patients with and without blocking antibodies was similar. We conclude that the currently available data are of critical importance in the interpretation of the obtained clinical experience and for further development of this therapeutic strategy.