The difficulty of working with the intact brain in vivo has led to the increasing use of nerve cell cultures in neurobiology. However, dissociated cells cannot be unambiguously identified by morphological criteria before the third week in culture, for it is not until then that the basic morphology and size of neurones become stable so that these and other cell types can be easily distinguished. However, cultured neurones can be identified by various cytochemical techniques based on (1) the detection of neurotransmitters or receptors for transmitters, (2) the presence of the Thy 1 antigen and the receptor for tetanus toxin, which are present on the membrane of most neurones, and (3) the presence in neurones of neurone-specific enolase (NSE), a cytoplasmic enzyme, which can only be identified on fixed specimens. Furthermore, other cell types in culture can also be specifically labelled. For instance, antisera to galactocerebroside bind selectively to oligodendrocytes, and antibodies to a neural tumour bind selectively to Schwann cells. We report here the selective interaction of phosphorylcholine-binding myeloma proteins (PC-BMP) with mouse neurones in culture and in suspension. Phosphorylcholine (PC) is found as part of lecithin and sphingomyelin molecules in variable amounts in eukaryotic and prokaryotic membranes, including plasma membranes.