The allophycocyanin (APC), crosslinked, goat anti—mouse IgG is prepared from affinity-purified antibodies that react with IgG heavy chains and all classes of immunoglobulin light chains from mouse. To minimize crossreactivity, it has been adsorbed against human IgG and human serum prior to conjugation. Because this phycobiliprotein is optimally excited at 633 nm but can also be excited at 594 nm, it can be used with samples simultaneously labeled with Texas Red dye or Alexa Fluor 594 dye without a second excitation source. APC can be used in flow cytometry and imaging applications and is more photostable than Cy5 conjugates.