Lithium is a immunomodulator that can increase immunoglobulins to confront infectious desese. It is also able to increase the production of interleukins, interferon gamma and prostaglandins. We conducted this study to evaluate the correlation between lithium consumption and humoral immunity. Blood samples were obtained from 76 patients under lithium treatment. We classified the patients according to their serum lithium level in to five categories. Using single radial immuno diffusion technique, we measured IgG, IgA, IgM, C3 and C4 levels in the blood samples of our patients and a group of 100 controls as well. Analysis of variance method was used for comparison of groups. We found significant differences between groups in level of IgG, IgA, IgM and C3 (P<0.05). This findings show that lithium has many effects on humoral immunity.