Recently in Japan, one form of vitamin B12, methylcobalamin also known as methyl B12, has attracted the attention of physicians as a therapy for patients with rheumatoid arthritis. However, its immunological actions in vivo are still unknown. In this study, we induced the in vitro production of such cytokines as interleukin-6 (IL-6), interferon-gamma (IFN-gamma), and interleukin-1 beta (IL-1 beta) by adding various mitogens (phytohemagglutinin:PHA, concanavalin A: ConA, or pokeweed mitogen:PWM) as well as recombinant interleukin-2, and we investigated the effects of methyl B12 (final concentration, 8-8,000 ng/ml) on the production of these cytokines by peripheral mononuclear cells. As compared to the controls, IL-6 production induced by PHA and ConA on Day 4 of the culture was suppressed by an average 60-70% when methyl B12 (80-8,000 ng/ml) was added to the medium. IFN-gamma production decreased dose-dependently with methyl B12, i.e., it decreased to 46% of the control when this production was induced by rIL-2, and decreased to 56-66% when it was induced by mitogens. The effect of methyl B12 on IL-1 beta production on Day I of the culture was small. These findings indicate that methyl B12 suppresses mainly the cytokine production of T lymphocytes. Such suppressive effects as shown in the in vitro situation are expected to be expressed also in vivo in patients with rheumatoid arthritis, especially at articulation lesion sites.