We previously reported cell cycle-dependent negative regulation of p185(neu) (decreased tyrosine phosphorylation and kinase activity, with electrophoretic mobility retarded by serine/threonine phosphorylation) in M phase and the escape of mutation-activated p185(neu*) from this regulation. Our present results showed that retardation of electrophoretic mobility occurs independently of the cells' transformed status. We found that normal p185(neu) lost its ability to dimerize in the M phase. We demonstrated a physical association between cdc2 (a serine/threonine kinase, active in M phase) and p185(neu). We showed that the carboxy terminal portion of p185(neu*) is phosphorylated in vitro by cdc2. Many phosphopeptides (at least three phosphoserine residues) unique to the M phase were identified, and the in vivo and in vitro phosphopeptide patterns were superimposable. In contrast, mutation-activated p185(neu*) dimerized in the M phase with no changes in electrophoretic mobility, failed to associate with cdc2 and no unique phosphoserine residues could be identified in the M phase (data not shown), consistent with the escape of p185(neu*) from cell cycle-dependent regulation. Our results suggest that this escape is an intrinsic property of the mutation-activated p185(neu*) independent of its ability to transform cells. Our results also suggest the involvement of serine/threonine kinases such as cdc2 in the cell cycle-dependent negative regulation of p185(neu).