Several studies have shown that kinases and phosphatases can interact with the centrosome during interphase and mitosis suggesting that centrosomal components might be the targets of these enzymes. The association of the cAMP-dependent protein kinase type II and the mitotic kinase p34cdc2 with centrosomes from human lymphoblast cells has previously been shown (Keryer et al, 1993, Exp Cell Res 204, 230-240; Bailly et al, 1989, EMBO J 8, 3985-3995). In this paper we demonstrate that isolated centrosomes are able to phosphorylate a few number of centrosomal proteins (M(r) 230-220000; 135000 and 50000) and also H1 histone. The phosphorylation of H1-histone is cell cycle dependent and modulated by phosphatases. The use of kinase and phosphatase inhibitors and the addition of the catalytic subunit of cAMP-dependent kinase or of cyclinB-p34cdc2 kinase showed that both kinases phosphorylate the same centrosomal substrates. In addition two centrosomal proteins (M(r) 100000 and 37000) were phosphorylated only by p34cdc2 kinase. Although the low amount of centrosomal proteins precluded a full characterization of these substrates we discuss the identity of the major centrosomal phosphoproteins by comparison with proteins known to associate with microtubule-organizing centres or mitotic spindles. Our results raise also the intriguing possibility that the cAMP-dependent protein kinase could be regulated by the mitotic kinase at the entry of mitosis.