The phosphoprotein (P protein) from human respiratory syncytial virus Long strain, labelled in vivo with [32P]orthophosphate, was purified from virions or virus-infected human epithelial (Hep-2) cells. The main phosphorylated amino acid found was serine. The determination of the N-terminal sequence of unphosphorylated and phosphorylated fragments of P protein obtained after chemical or enzymic treatments suggested that some or all of the six serines present at positions 116, 117, 119, 143, 156 and 161 are the major phosphorylated residues, although a modification in serine residues at positions 86, 94 and 99 can not be ruled out.