A new satellitism test designed to facilitate the isolation and identification of Haemophilus influenzae and Haemophilus parainfluenzae is described. In the basal medium, nicotinamide adenine dinucleotide is incorporated at a concentration of 0.2 mug per ml, an amount adequate for H. influenzae but not for H. parainfluenzae. Two disks are placed on the surface of the medium, one disk being impregnated with 60 mug of hemin and the other with 15 mug of nicotinamide adenine dinucleotide. Under these conditions, H. influenzae strains grow around the hemin disk only and the majority of H. parainfluenzae grow around the nicotinamide adenine dinucleotide disk. This procedure gives results which are more clear cut than other established methods, especially in sputum culture.