Preparative isoelectric focusing was used to separate free bacterial NAD+ nucleosidase from its complex with a bound host component. Both fractions were characterized by optimum temperature and activation energy of denaturation. The bacterial product is enzymically inactive. The enzymically active structure is formed upon binding to the host component. Only the host organism can provide the suitable, activating structure. The host component in the present system is added to the cultivation medium with a beef heart extract but it can be replaced by serum albumin. The possible role of albumin as a carrier structure for flexible and enzymically inactive peptides is discussed. Different peptides bound to albumin can provide different enzyme activities. The term binary enzyme is coined, referring to a situation where the two enzyme components are coded at genetically distant loci. The pathogen makes use of the carrier structure of albumin type and produces another polypeptide invested with an enzyme activity convenient for the pathogen.