An indirect fluorescent antibody technique which allows the detection of antibody against or the recognition of Haemophilus ducreyi has been developed. Heat inactivated antiserum was reacted uith prepared smears of Haimophilus species and selected organisms commonly associated with penile ulcerations. In serum absorbed with Haemophilus sp no fluorescence was observed with 156 heterologous organisms, but all six strains of bacteriologically confirmed H. ducreyi gave significant reactions with the absorbed anti-WD-68 serum.