Plasmids carrying the herpes simplex virus thymidine kinase (tk) gene were used to transfect thymidine kinase-deficient cells of the mouse fibroblast cell line LM(tk-). Individual cell clones were cultivated in selective hypoxanthine-aminopterin-thymidine medium to determine the number of integrated plasmid copies which was almost always in the range of one to three copies per genome. In contrast, cells transfected with plasmids carrying a promoterless "truncated" tk gene typically contained between 10 and 25 copies per genome. Surprisingly, when the truncated tk gene was transfected together with a simian virus 40 DNA segment, including its transcriptional enhancer, the number of integrated tk gene copies was always low, between one and three copies per genome. We have analyzed the genomic organization of integrated truncated tk genes by blot hybridization of restricted cellular DNA and concluded that integrated units of plasmid DNA molecules are arranged in tandem arrays which remain stable in most cases for many cell generations. In only 1 of ca. 20 cell clones did we observe a retraction and expansion of the number of integrated promoterless tk genes as a response to the removal or readdition of selective pressure. Surprisingly, the thymidine kinase activity determined in extracts from cells growing in selective hypoxanthine-aminopterin-thymidine medium (high numbers of integrated tk gene copies) was nearly the same as the enzymatic activity in cells growing in nonselective medium (low copy numbers). Moreover, Northern blots of polyadenylated RNA, extracted from cells growing under selective and nonselective conditions, showed that, in both cases, the major species of tk-specific transcripts was ca. 1.5 kilobases in size, as expected for a tk-specific mRNA containing the entire coding region of the gene. Thus, disproportionate DNA replication appeared not to be essential for an active tk gene expression in these cells. We discuss possible pathways leading to the formation of tandem arrays of integrated truncated tk genes and the conditions required for disproportionate DNA replication in the unique case in which we found a retraction and expansion of tk gene copy numbers as a response to selective growth conditions.