Experiments were conducted under greenhouse and field conditions to determine the effects of Mo and S applications on the Mo and Cu concentrations of timothy, alfalfa, and red clover. Applications of S decreased the Mo concentrations of tissues of all crops. A significant interaction between Mo and S was observed, whereby the decreases in Mo concentration of tissue with applied S were more marked when Mo was added than in the absence of applied Mo. Without applied Mo, the Mo concentration of tissues was higher under field than under greenhouse conditions. The total S concentration of timothy and red clover tissues ranged from 0.15 to 0.21 percent and was not affected by the applications of S or Mo. Copper concentrations in crop tissues of greater than 5.7 ppm, associated with 0.15 to 0.21 percent S in tissues, with less than 0.5 ppm Mo applied to the soil, as reported in this paper are considered to be safe from the animal nutrition standpoint. Levels of tissue Mo resulting from application of 0.5 to 1.0 ppm Mo, which were reported as toxic from an animal nutrition standpoint, were corrected by applications of 50 to 200 ppm S. In the second and subsequent cuts, 50 ppm applied S was adequate in alleviating Mo toxicity at all levels of applied Mo. Under greenhouse conditions, yield response to added Mo as obtained on red clover was related to tissue Mo concentrations of 0.22 ppm.