A series of antisense pentadecamers complementary to a variety of target sequences between the cap and AUG initiation codon regions of c-myc mRNA was synthesized and used to treat human promyelocytic leukemia HL-60 cells. The sensitivity of the cap-region sequences to antisense inhibition of c-myc p65 expression was two to three times that of the original initiation codon antisense sequence. The other target sequences downstream of the cap and up to the AUG initiation codon were comparable to the initiation codon sequence, except that the first splice junction was slightly more sensitive. At the primary initiation codon target, a dodecamer was about half as effective as the original pentadecamer, whereas an octadecamer was about twice as effective. The observation of variation in antisense efficacy as a function of target location in c-myc mRNA may represent a combination of the effects of hybrid arrest, RNase H attack, and interference in RNA processing. Alternatively, the most sensitive targets might be those that are the most exposed in the secondary and tertiary structure of c-myc mRNA.