 EC20 concentrates in the extremities and liver and spleen of rats with adjuvant induced arthritis. The increased uptake is mediated by overexpressed folate receptor on macrophages, and the intensity of the uptake is greatly reduced by addition of excess folate, supporting the specificity of the uptake. Depletion of macrophages greatly reduced folate receptor and abolishes uptake of EC20. This suggests that EC20 may be useful in assessing macrophage involvement in the active inflammatory process of RA, and perhaps other systemic immune mediated disorders as well. 