Burkitt lymphoma cells carrying either a rearranged or unrearranged c-myc oncogene were examined with the use of probes from the 5' exon and for the second and third exon of the oncogene. The results indicate that the normal c-myc gene on chromosome 8 and the 5' noncoding and 3' coding segments of the c-myc oncogene separated by the chromosomal translocation are under different transcriptional control in the lymphoma cells. Burkitt lymphoma cells carrying a translocated but unrearranged c-myc oncogene express normal c-myc transcripts. In contrast, lymphoma cells carrying a c-myc gene rearranged head to head with the immunoglobulin constant mu region gene express c-myc transcripts lacking the normal untranslated leader.