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The regulation of histone gene expression was examined after confluent, nondividing WI-38 human diploid fibroblasts were stimulated to proliferate. Histone mRNA sequences were assayed by hybrid formation with an 3H-labeled single-stranded DNA complementary to histone mRNAs. Histone mRNA became associated with polyribosomes concomitant with the activation of DNA synthesis. The ability of chromatin from WI-38 cells to serve as a template for in vitro transcription of histone mRNA sequences parallels the onset of DNA replication. A role for nonhistone chromosomal proteins in the control of histone gene readout is suggested because, when chromatin from confluent WI-38 cells was dissociated and then reconstituted in the presence of S phase nonhistone chromosomal proteins, a 500-fold activation of histone mRNA sequence transcription was observed.