Brown cells of the red gland in Mercenaria mercenaria are involved in the excretory process. Their involvement in the removal of foreign substances from the tissues of Mercenaria was studied. Toluidine blue (soluble dye) accumulated in brown cells after dye injection into the foot muscle and after addition to brown cell isolates. In comparison, carmine red (particulate dye) did not accumulate under the same conditions. FITC-labeled bovine albumen accumulated in brown cells when introduced in vivo and to cell isolates. Kinetic studies indicated uptake of FITC-albumen occurred within 2 h when added to cell isolates and within 24 h after injection into the foot muscle. Brown cell isolates were separated into three fractions on a discontinuous Percoll gradient. Fraction 1 contained primarily what appeared to be precursor brown cells. The majority of the cells in fraction 2 were brown cells. Fraction 3 was entirely brown globular material. Assays indicated the presence of acid phosphatase, glutathione reductase and lysozyme in lysates from each fraction. Thus it appears that brown cells of the red gland contain enzymes involved in detoxification and degradative processes. (Copyright (c) 1990 Elsevier Science Publishers Ltd., England.)