The 105-kDa heat shock protein alpha (HSP105 alpha) and HSP105 beta are mammalian heat shock proteins that belong to the HSP105/HSP110 family. Both HSP105a and HSP105 beta consist of acidic and basic isoforms. Here we report that the acidic isoforms are serine phosphorylated HSP105 alpha or HSP105 beta. Furthermore, using an in-gel kinase assay with HSP105 alpha or HSP105 beta as the substrate, the protein kinase that phosphorylates HSP105 alpha and HSP105 beta was identified as casein kinase II. Since phosphorylated HSP105 alpha is especially prominent in the brain compared to other tissues of mice and rats, the phosphorylation of HSP105 alpha by casein kinase II may be biologically significant.