Activated SureLight(R) Allophycocyanin (SL-APC) is a highly fluorescent phycobiliprotein which has been chemically activated for easy conjugation to reduced immunoglobulins or other sulfhydryl-containing proteins. The SL-APC has been activated with SMCC (succinimidyl 4-[N-maleimidomethyl]-cyclohexane- 1-carboxylate) under conditions that result in modification of only a few lysine residues on each SL-APC molecule. The reactive group introduced by activation is maleimide, which readily reacts with reduced cysteines and other sulfhydryl residues under mild conditions of temperature and pH.