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The binding of immune complexes to immobilized bovine conglutinin (K) was studied in order to develop an ex vivo immunoadsorbent system for the removal of circulating immune complexes. K was immobilized to the surface of solid, polymeric beads of two different sizes (1–3 um and 38–63 um diameter) which were activated with N-hydroxysuccinimide ester groups. The binding of both a model immune complex, aggregated human globulin (AHG) and an actual immune complex, bovine serum albumin (BSA): anti-BSA, to the immobilized K were determined utilizing both batch and flow conditions. The binding of AHG occurred rapidly (within 15 min.) at 37°C and additional binding was attained by using a subsequent low temperature incubation (37°C, 30 min., followed by 4°C, 18 hrs.). Uptake of AHG from solution was 2.6 g/g immobilized K. The binding of the 125I-AHG to immobilized K was found to be linear with concentration over the range of 20-2,000 ug AHG/ml tested. Similarly, the binding of BSA:anti-BSA complexes in solution to immobilized K was 0.16 ug 125I-BSA:anti-BSA/ug immobilized K. Significant amounts of the bound AHG were found to be released nonspecifically in the presence of buffered protein solutions, including human serum albumin (HSA) and BSA, while bound BSA:anti-BSA complexes were not released by similar treatments. Effluents of solution perfused over immobilized K showed no toxicity upon intravenous infusion into mice. These studies suggest that K may be immobilized on a biocompatible solid support and in this state retains functional capacity to extract immune complexes from solution. Hence, this may represent a promising system for use as an extracorporeal immunoadsorbent to remove circulating immune complexes in vivo.