The nucleotide sequence of an 8.2-kb DNA fragment from the 5' proximal part of the chicken myb proto-oncogene spanning 1761 nucleotides upstream and 6436 nucleotides downstream from a presumed c-myb initiation codon was determined. A 3.3-kb G + C-rich region found in this sequence had also other features characterizing CpG islands, i.e. no CpG underrepresentation and lack of CpG methylation. In haematopoietic tissues c-myb mRNA synthesis starts in two major regions of the CpG island, namely 98 to 108 and 143 to 145 nucleotides upstream from the c-myb initiation codon. These two regions are in or close to the 124-bp evolutionarily conserved element located in the middle part of the CpG island. No alternative splicing of the 5' end of c-myb mRNA suggested earlier (1,2) was observed. The c-myb promoter contains neither TATA nor CAAT box-like structures at the usual positions. Instead, numerous potential Sp1 factor binding sites were found both upstream and downstream from the transcription initiation sites. Moreover, consensus v-myb protein DNA-binding sites were revealed in the promoter region and in sequences downstream from it.