The localization of neurofilament triplet proteins in PC12 cells grown in the absence of (PC12-) or maintained in the presence of (PC12+) nerve growth factor (NGF) was studied using indirect immunofluorescence and monospecific, immunosorbent purified antibodies to 68,000 (P68), 150,000 (P150) and 200,000 (P200) dalton neurofilament proteins. The intensity of immunofluorescent staining of the triplet proteins was always greater in PC12+ compared with PC12-cells. Neuritic staining was seen in PC12+ cells with all 3 monospecific antibodies to neurofilament proteins. However, the perikaryal distribution of each of the neurofilament proteins differed in both PC12+ and PC12-cells. Monospecific antibodies to P68 protein yielded a 'ball-like' cytoplasmic staining pattern whereas monospecific antibodies by P150 protein stained in a stippled pattern. Monospecific antibodies to P200 on the other hand diffusely stained the perikaryal cytoplasm with very faint but detectable foci of 'ball-like' configurations and stippling. Electron microscopic study of PC12+ and PC12-cells revealed intermediate filaments in the cell bodies of both as well as in the processes of the former. 'Ball-like' clusters of such filaments were rarely seen. However, these filaments lacked the three-dimensional organization typical of intact neurofilaments. It is concluded that PC12 cells contain dissociated or incompletely assembled immunoreactive neurofilament triplet proteins and that these proteins can be induced by NGF. The PC12 cells are therefore an attractive model system not only for studies of neuronal differentiation but also for studies of neurofilament metabolism and disorders thereof.