A method for evaluating the aggregation of a proteinaceous material in a suspension comprising an organopolysiloxane, comprising: (a) providing an aqueous suspension of a fluorescently-labeled organopolysiloxane and a fluorescently-labeled proteinaceous material wherein the organopolysiloxane of said fluorescently-labeled organopolysiloxane has a viscosity of from 100•10 -6 m 2 /s to 1,000,000•10 -6 m 2 /s (100 to 1,000,000 centistokes), wherein said organopolysiloxane of said fluorescently-labeled organopolysiloxane is selected from at least one of: - alkyl-substituted organopolysiloxanes, - organopolysiloxanes comprising at least one alkenyl group selected from vinyl, allyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, and decenyl; - organopolysiloxanes comprising at least two polar groups selected independently from acrylate, methacrylate, amino, imino, hydroxyl, epoxy, ester, alkyloxy, isocyanate, phenolic, polyurethane oligomeric, polyamide oligomeric, polyester oligomeric, polyether oligomeric, polyol, and carboxypropyl; - organopolysiloxanes comprising one or more fluoro groups, - cyclic siloxanes; - organopolysiloxanes comprising at least two pendent hydrogen groups, and - hydroxyl functional siloxanes; and wherein the fluorescently-labeled proteincaceous material comprises at least one proteinaceous material which is a monoclonal antibody; (b) measuring relative particle fluorescence intensity of the fluorescently-labeled organopolysiloxane and the fluorescently-labeled proteinaceous material using fluorescence-activated particle sorting; and (c) comparing the relative intensity of the fluorescently-labeled organopolysiloxane to the relative intensity of the fluorescently-labeled proteinaceous material. (a) providing an aqueous suspension of a fluorescently-labeled organopolysiloxane and a fluorescently-labeled proteinaceous material wherein the organopolysiloxane of said fluorescently-labeled organopolysiloxane has a viscosity of from 100•10 -6 m 2 /s to 1,000,000•10 -6 m 2 /s (100 to 1,000,000 centistokes), wherein said organopolysiloxane of said fluorescently-labeled organopolysiloxane is selected from at least one of: - alkyl-substituted organopolysiloxanes, - organopolysiloxanes comprising at least one alkenyl group selected from vinyl, allyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, and decenyl; - organopolysiloxanes comprising at least two polar groups selected independently from acrylate, methacrylate, amino, imino, hydroxyl, epoxy, ester, alkyloxy, isocyanate, phenolic, polyurethane oligomeric, polyamide oligomeric, polyester oligomeric, polyether oligomeric, polyol, and carboxypropyl; - organopolysiloxanes comprising one or more fluoro groups, - cyclic siloxanes; - organopolysiloxanes comprising at least two pendent hydrogen groups, and - hydroxyl functional siloxanes; and wherein the fluorescently-labeled proteincaceous material comprises at least one proteinaceous material which is a monoclonal antibody; - alkyl-substituted organopolysiloxanes, - organopolysiloxanes comprising at least one alkenyl group selected from vinyl, allyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, and decenyl; - organopolysiloxanes comprising at least two polar groups selected independently from acrylate, methacrylate, amino, imino, hydroxyl, epoxy, ester, alkyloxy, isocyanate, phenolic, polyurethane oligomeric, polyamide oligomeric, polyester oligomeric, polyether oligomeric, polyol, and carboxypropyl; - organopolysiloxanes comprising one or more fluoro groups, - cyclic siloxanes; - organopolysiloxanes comprising at least two pendent hydrogen groups, and - hydroxyl functional siloxanes; and (b) measuring relative particle fluorescence intensity of the fluorescently-labeled organopolysiloxane and the fluorescently-labeled proteinaceous material using fluorescence-activated particle sorting; and (c) comparing the relative intensity of the fluorescently-labeled organopolysiloxane to the relative intensity of the fluorescently-labeled proteinaceous material.