The potential function of neuronal tau was found by our recent studies on the effect of tau on the melting temperature of both calf thymus DNA and plasmid pBluescript-II SK (Hua and He, Chin. Sci. Bull. 2000, 45:999-1001). Herein we examined whether or not the interaction of tau with DNA was related to phosphorylation and aggregation. Tau, phosphorylated by neuronal cdc2-like kinase, associated with DNA as shown by electrophoretic mobility shift assay. Similar to native tau, phosphorylated tau could increase the melting temperature of calf thymus DNA. When tau was aggregated or treated with formaldehyde, neither native tau nor phosphorylated tau kept its ability to interact with DNA, suggesting that binding of tau to DNA was in an aggregation-dependent, and a phosphorylation-independent, manner.