The distribution of microtubule-associated protein lB (MAPlB) phosphorylated by either proline-directed protein kinase (PDPK) or casein kinase II (CK II) in neuroblastoma cells and hippocampal neurons has been studied by immunofluorescence using specific antibodies to distinct phosphorylation-sensitive epitopes. A proximo-distal gradient of increasing PDPK-catalyzed phosphorylation of MAPlB is superimposed on a proximo distal gradient of decreasing CK II-catalyzed MAPlB phosphorylation within growing axon-like neurites. Additionally, CK II-phosphorylated MAPlB is present in cell bodies and dendrites where no PDPK-phosphorylated MAPlB is observed. These results suggest distinct roles for both types of modifications of MAPlB in developing neurons.