Previous studies have shown that the molecules of P protein associated with transcriptionally active Sendai virus nucleocapsids are arranged in discrete clusters. Our study investigates whether or not this localized distribution is due to the existence of only a few P protein binding sites on the nucleocapsid core. We used immunoelectron microscopy to examine whether additional P proteins could bind at locations between the groups of endogenous P proteins. To differentiate between endogenous and added proteins, we constructed a recombinant gene which instructs the in vitro synthesis of a chimeric protein containing the carboxyl-terminal nucleocapsid-binding region of P protein, fused to chloramphenicol acetyltransferase (CAT). Immunogold labelling, using an antibody to the CAT moiety, revealed at the electron microscope level, that the chimeric product bound to nucleocapsids at many sites located over the entire length of the nucleocapsid. This indicated that the localized distribution of P protein molecules is not due to a limited number of P protein binding sites on the nucleocapsid core.