LSP1 is the major substrate for mitogen-activated protein kinase-activated protein kinase 2 in human neutrophils. . Inspection of the human LSP1 protein sequence (22) identifies two serine residues at positions 204 and 252 as potential phosphorylation sites. The results show that LSP1 is a substrate for MAPKAP kinase 2 in vitro and that the phosphorylation site(s) are located in the C-terminal 152 amino acid residues, as predicted from the sequence.