We have shown earlier that certain proline-directed kinases such as MAP kinase or GSK-3 can phosphorylate tau protein in an abnormal manner reminiscent of tau from Alzheimer paired helical filaments [Drewes et al. (1992); Mandelkow et al. (1992)]. Both kinases are abundant in brain tissue and associate physically with microtubules through several cycles of assembly and disassembly. In this report we show that cdk2/cyclin A incorporates = 5 Pi into recombinant tau, and that it also induces the MR shift and antibody reactivity typical of Alzheimer tau. However, since there is no cdk2 in brain [Meyerson et al. (1992)] we looked for other members of this family of kinases. Using an antibody against the conserved N-terminus we isolated a cdk-like kinase from brain which was capable of inducing the Alzheimer-like characteristics in tau by phosphorylation. Its size (31 kDa), target specificity (proline-directed), chromatographic behavior, and abundance in brain suggest that this kinase is similar or identical to the neuronal cdc2-like kinase nclk alias PSSARLE or cdk5 [Hellmich et al. (1992); Meyerson et al. (1992); Xiong et al. (1992); Tsai et al. (1993)]. This was confirmed by an antibody specific for cdk5. Like MAP kinase and GSK-3, this kinase is physically associated with microtubules and can be enriched by cycles of microtubule assembly and disassembly. Thus, cdk5 should be regarded as another kinase that could be held responsible for the changes in tau protein during Alzheimer disease progression.