Adenovirus 12 mRNA's transcribed from the transforming region were analyzed and mapped on viral DNA by the nuclease S1 gel and diazobenzyloxymethyl paper blot techniques in cells transformed of adenovirus 12 DNA and in cells early and late after lytic infection with adenovirus 12. Two initiation sites of mRNA transcription and two kinds of splicing were found in each of early regions 1A and 1B. For early region 1A mRNA's, four species were found in lytically infected cells. Three of them were commonly found in cells transformed by either HindIII-G or EcoRI-C. Cells transformed by HindIII-G contained two additional 1A transcripts, which could be the 3' portions of chimeric mRNA's of cellular-viral or viral-viral sequences. Transcription in the 1B region diverged among the above cell lines. Early after lytic infection, no appreciable amount of 1B mRNA was detected, whereas two species of mRNA's, one corresponding to protein IX mRNA and another having splicing, were found at the late stage. In cells transformed by EcoRI-C, a distinct mRNA species with splicing was observed. Cells transformed by HindIII-G contained a transcript from the leftmost part of the 1B sequence at the 5' portion of chimeric mRNA species, suggesting the presence of tandem integration of viral DNA in the cells. Other mRNA species in the 1A and 1B regions were also detected in both transformed cell lines. The results are discussed in relation to the nucleotide sequence of the HindIII-G fragment of adenovirus 12 DNA.