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2547 Background: Treatment of CRC patients (pts) with the humanized mAb IGN311 targeting the carbohydrate Lewis Y eliminated circulating tumor cells in blood and thereby confirmed the clinical profile of the parent murine antibody ABL364, which showed elimination of Lewis Y and cytokeratin positive cells in bone marrow of pts with breast cancer. Methods: An open-label, single treatment arm, uncontrolled study with IGN311 (100mg per dose, intravenously on day 1 and 7) in pts with malignant effusion (ascites or pleural effusion) is being conducted with the primary objective to examine safety and tolerability. Secondary objectives are volumetric measurement of the malignant effusion and to obtain data for several immunological parameters. Results: 4 pts (2 pts with gastric cancer and malignant ascites, 2 pts with breast cancer and malignant pleural effusion / ascites) have completed the study until December 2005. IGN311 was well tolerated with only one patient showing up to grade 2 nausea, vomiting and skin rashes as side effect after 1st application which was easily managed. In all pts significant levels of IGN311 were measured followed by an increase in CD45 positive cells in the effusion. The patient with the highest level of Lewis Y expressing tumor cells showed a reduction of effusion volume during treatment. Conclusions: IGN311 was well tolerated, permeated into malignant effusion and attracted immune cells leading to decreased tumor cell counts in the effusion. In the case of strong Lewis Y expression of malignant cells in the effusion a reduction of the effusion volume could be demonstrated. [Table: see text]