MAP kinase‐activated protein kinase‐2 (MAPKAP kinase‐2) phosphorylates the serine residues in murine heat shock protein 25 (hsp25) and human heat shock protein 27 (hsp27) which are phosphorylated in vivo in response to growth factors and heat shock, namely Ser¹⁵ and Ser⁴⁰ (hsp25) and Ser¹⁵, Ser⁷⁸ and Ser⁸² (hsp27). Se⁸⁶ of hsp25 and the equivalent residue in hsp27 (Ser⁸²) are phosphorylated preferentially in vitro. The small heat shock protein is present in rabbit skeletal muscle and hsp25 kinase activity in skeletal muscle extracts co‐purifies with MAPKAP kinase‐2 activity throughout the purification of the latter enzyme. These results suggest that MAPKAP kinase‐2 is the enzyme responsible for the phosphorylation of these small heat shock proteins in mammalian cells.