Ribosomal protein phosphorylation was investigated in Ehrlich ascites tumor cells infected with vaccinia virus (Copenhagen strain). After 90 rnin of simultaneous infection and 32P-labelling, ribosomal proteins Sa, S2 and S13 appear specifically phosphorylated as well as Sb/La, P1 and S6, which are also phosphorylated in control cells. Sa is an acidic protein, whose phosphorylation has not been observed previously. A kinetic study showed that S2 is phosphorylated very rapidly within 10 min after the beginning of infection and it is complete 1 h later. The phosphorylation of S13 begins after a lag time of about 1 h and is completed after about 2.5 h of infection. Moreover only one phosphate is incorporated into S13 on a serine residue while up to four phosphates are incorporated into S2, the first on a serine and the three following on threonine residues. In vivo experiments, carried out in the presence of cycloheximide and cordycepin, suggest a viral origin for the kinase involved in the phosphorylation of S2 and S13. Moreover, in vitro experiments demonstrated that the kinase associated with the viral cores is capable of phosphorylating S2 on a serine residue only. In our cell/virus system, no significant difference in S6 phosphorylation was detected, when compared to uninfected cells. It is concluded that the specific and efficient phosphorylation of three ribosomal proteins from the 40 S ribosomal subunit correlate well with possible translational mechanisms ensuring the efficient expression of early and late genes of vaccinia virus. In the light of these and previous results [Person, A. and Beaud, G. (1986) J. Biol. Chern. 261, 8283-82891, a mechanism is proposed for the shut-off of host protein synthesis and the selective translation of mRNAs of viral origin.Infection of cells with vaccinia virus, a DNA virus replicating in the cytoplasm [l], results in a shut-off of host protein synthesis concomittant to the translation of the viral early mRNAs transcribed from vaccinia cores. It was shown by Kaerlein and Horak that several ribosomal proteins of HeLa cells are modified after infection with vaccinia virus (WR strain) and they suggested a possible role of the phosphorylation of S2, S6 and S16 in the establishment of the shut-off of host protein synthesis [2, 31. These authors gave evidence that the protein kinase known to be associated with vaccinia virions [4-71 may play a role in the phosphorylation of the ribosomal proteins but an early virus-induced enzyme was also responsible for this effect [3].A protein-synthesis inhibitor, associated with vaccinia virions, has been recently purified yielding a 1 1-kDa basic protein [8]. This inhibitor is clearly different from vacciniaassociated protein kinase and is likely to be responsible for the shut-off of protein synthesis shown to occur in the absence of viral gene expression in several cell/vaccinia virus systems. Thus it has been supposed that, in a productive infection, a viral-coded protein is produced in sufficient amounts to re-