Phosphorylation of microtubule-associated protein MAP1B and the neuronal-specific beta III-tubulin isoform takes place during neurite growth in neuroblastoma cells. Protein kinase CK2 (formerly referred to as casein kinase 2) is possibly involved in beta III-tubulin phosphorylation. As for MAP1B, there are at least two types of phosphorylation; one catalyzed by proline-directed protein kinases and another catalyzed by CK2. Protein kinase CK2 is primarily localized to the nuclei in proliferating neuroblastoma cells, whereas an increased amount of the enzyme is present in the cytoplasm of postmitotic cells bearing neurites. Treatment of neuroblastoma cells with an antisense oligonucleotide which specifically results in CK2 catalytic subunit depletion inhibits neuritogenesis. CK2 depletion is accompanied by dephosphorylation of MAP1B on the corresponding phosphorylatable sites. This dephosphorylation is paralleled by a release of MAP1B from microtubules. These results suggest that MAP1B phosphorylation by CK2 may be required for the assembly of microtubules within neurites. Other neuronal cytoskeletal proteins including MAP1A and tau are also substrates for CK2, indicating a role for the enzyme in the regulation of cytoskeletal functions also in mature neurons.