The in vitro phosphorylation of chicken desmin by the catalytic subunit of cAMP-dependent protein kinase was analysed. Phosphorylated desmin loses the ability to form intermediate filaments (IFs). Fragmentation at the sole cysteine and mild chymotryptic treatment show a differential phosphorylation of the three structural domains. Only the amino-terminal head domain is the target of the kinase. Peptide analysis shows that serine 29 is fully phosphorylated, while serine 35 and 50 are phosphorylated at least at 22 and 50% respectively. All three sites show the sequence arginine-X-serine with X being a small residue. These results strengthen the view that the nonhelical head domain has a strong influence on filament integrity most likely via a direct influence of some of its arginine residues. Taken together with previous results (Inagaki et al., 1987) on the phosphorylation of vimentin by kinase A, a new view on IFs emerges. Phosphorylation could allow for regulatory processes in assembly and turnover.