In rat brain, the microtubule-associated protein 2 (MAP2) contains two low molecular weight isoforms, consisting of 467 and 498 amino acids and designated MAP2c and d, respectively. The present study describes the expression of these isoforms in Escherichia coli and their subsequent purification to homogeneity. On SDS-polyacrylamide gel electrophoresis, the recombinant proteins ran with apparent molecular masses of 69 and 74 kDa. A microtubule-assembling assay demonstrated that the recombinant protein was biologically active and that the tubulin polymerization rate for MAP2d was twice as fast as that for MAP2c. After dephosphorylation with alkaline phosphatase, preparations of low molecular weight MAP2 from fetal rat brain aligned with recombinant MAP2c. Moreover, recombinant MAP2c could be phosphorylated with a brain extract kinase activity, resulting in a reduced electrophoretic mobility similar to that observed in fetal MAP2.