Sporadic reports have appeared that suggest silicon plays a functional role in immune function by affecting lymphocyte proliferation. In addition, there is also considerable interest in supplemental arginine as a modulator of immune function. Therefore, the purpose of this animal experiment was to determine the effect of supplemental compared to adequate arginine on immune function as measured by splenic T-lymphocyte proliferation in the presence of adequate or inadequate dietary silicon. The independent variables were, per gram of fresh diet, silicon supplements of 0 or 35 microg and arginine supplements of 0 or 5 mg. The basal diet contained 2.3 microg silicon/g and 7.82 mg L-arginine/g. After feeding the male rats (nine per treatment group) for 8 wk, spleen lymphoid cells were isolated and cultured with methyl-3[H]thymidine. Supplemental arginine significantly decreased Con-A-induced DNA synthesis of splenic T-lymphocytes, but the response to arginine was influenced by dietary silicon. The decreased DNA synthesis was more marked when rats were fed adequate silicon than when fed inadequate silicon. Also, when arginine was not supplemented, DNA synthesis was higher in lymphocytes from rats fed an adequate silicon diet than rats fed the inadequate silicon diet. These findings support the huypothesis that an interaction between silicon and arginine affects immune function and that inadequate dietary silicon impairs splenic lymphocyte proliferation in response to an immune challenge.