The CPK APC Labeling Kit is optimized for use in the conjugation of allophycocyanin (APC) to antibodies. APC can be isolated from various species of red or blue-green algae, each producing slightly different forms of the molecule. It is an ultra-sensitive, near-infrared fluorescent tracer with a high quantum yield (Ex/Em = 650 nm/660 nm). APC- labeled antibodies are used in applications such as flow cytometry and immunofluorescence. The CPK APC Labeling Kit contains SH-reactive APC. SMCC modified allophycocyanin reacts with the thiol groups of target antibody without the need for additional activation, thus simplifying the conjugation protocol. The CPK APC Labeling Kit contains a chemically cross-linked APC (CL-APC) that is much more stable than the native APC, but still retains the original spectroscopic properties. The amount of APC supplied in this kit is sufficient for labeling up to 1 mg of antibody. The kit provides all reagents, purification columns and a detailed step-by-step protocol. Cross-linked and SMCC-activated APC are also available as separate products.