R-phycoerythrin (R-PE) was extracted from red algae, Gracilaria fisheri from Pattani Province, Thailand, with 50 mM sodium phosphate buffer, pH 7.0, followed by precipitation with 30-50% final concentration of saturated ammonium sulphate solution at 0 degree C. The precipitate was further purified by DEAE-cellulose (DE-52) column chromatography. The purified R-PE showed a single band of Mr 240 kDa by polyacrylamide gel electrophoresis with the maximum absorption and maximum fluorescence emission at 565 nm and at 573 nm respectively, and the OD ratio of 565 to 280 nm was 6.7. The IgG fraction of a murine monoclonal antibody (Eh208C2-2 MIgG) raised against trophozoites of HM-1: IMSS strain of pathogenic E. histolytica was conjugated with purified R-PE by using the heterobifunctional compound N-succinimidyl3-(2-pyridyldithio)propionate (SPDP). The conjugate was shown by direct immunofluorescent antibody (DIFA) assay to stain specifically both the culture-derived and stool-derived E. histolytica trophozoites.