Using a 3H-labeled single-stranded complementary DNA probe for detection of histone mRNA sequences (Thrall, CL., Park, WD., Rashba, HW., Stein, JL.,Mans, RJ., and Stein, GS.(1974), biochem. Biophys. Res. Commun. 61,1443) we have found that histone genes are transcribed in vitro from chromatin isolated from S-phase HeLa cells but not from chromatin isolated from G1-phase cells (Stein, G., Park W., Thrall, C., Mans, R., and Stein, J. (1975a), Nature (London) 2578 764; Stein, G., Park, W., Thrall, C., Mans, R., Steins, J.(1975b), Biochem. Biophys. Res. Commun. 63, 945). Utilizing the technique of chromatin reconstitution, we have recently demonstrated that it is the nonhistone chromosomal protein portion of the genome that is responsible for this difference in in vitro histone gene expression (Stein et al., 1975a). In order to determine whether this is attributable to some component of the S-phase chromosomal proteins that promotes the transcriptin of histone genes, a component of the G1 phase chromosomal proteins that inhibits histone gene transcription, or both, in the present study chromatin from both G1 and S-phase cells was dissociated and then reconstituted in the presence of various chromosomal proteins. The results of this study confirm that it is the nonhistone chromosomal proteins that are responsible for the cell cycle stage specific differences in in vitro histone gene expression and further show that these differences can be accounted for by a component or components of the S-phase nonhistone chromosomal proteins that has the capacity, when reconstituted in the presences of G1 phase chromatin, to render the histone genes transcribable in a dose-dependent fashion.