The expression of late SPP1 genes depends on preceding SPP1 DNA replication. This is shown in nonpermissive infection with a mutant defective in DNA replication and after inhibition of DNA synthesis by HPUra. The potential for host gene expression is not significantly influenced by SPP1 infection, as evidenced by the continuation of host protein synthesis and the inducibility of glycerolphosphate dehydrogenase after infection. The involvement of a positive control element in the regulation of SPP1 gene expression is deduced from the observation that chloramphenicol prevents the synthesis of the only class of mRNA which is transcribed from the L-strand.