Members of the mitogen-activated protein (MAP) kinase family are implicated in mediating entry of cells into the cell cycle, as well as passage through meiotic M phase. These kinases have attracted much interest because their activation involves phosphorylation on both tyrosine and threonine residues, but little is known about their physiological targets. In this study, two distinct members of the MAP kinase family (p44mpk and p42mapk) are shown to phosphorylate chicken lamin B2 at a single site identified as Ser16. Moreover, these MAP kinases cause depolymerization of in-vitro-assembled longitudinal lamin head-to-tail polymers. Ser16 was previously shown to be phosphorylated during mitosis in vivo, and to be a target of the mitotic protein kinase p34cdc2 in vitro. Accordingly, lamins were proposed to be direct in vivo substrates of p34cdc2. This proposal is supported by quantitative analyses indicating that lamin B2, when assayed in vitro, is a substantially better substrate for p34cdc2 than for MAP kinases. Nevertheless, a physiological role of MAP kinases in lamin phosphorylation is not excluded. The observation that members of the MAP kinase family display sequence specificities overlapping that of p34cdc2 raises the possibility that some of the purported substrates of p34cdc2 may actually be physiological substrates of MAP kinases.