The sequence specificity of bleomycin damage was investigated utilising 340 bp alpha-DNA (a middle repetitive sequence in the human genome) as a target sequence. The following significant facts were found:- i) The dinucleotides GT and GC were cleaved on all occasions, GA most of the time, and AT, AC, GG and AA cleaved some of the time; ii) The base immediately 5' to the purine-pyrimidine dinucleotides was found to be statistically highly significant in determining the degree of damage caused by bleomycin, while other nearest neighbour bases had no significant effect; iii) The sequence specificity of bleomycin damage was determined on both strands and it was found that damage on either strand follows the above dinucleotide preference and is independent of the extent of damage on the opposite strand; iv) Bleomycin damage was compared between genomic 340 bp alpha-DNA and a cloned alpha-DNA with eleven base substitutions relative to the "consensus" sequence. There were forty-nine detectable differences in intensity of damage between these two DNA molecules. Although four of the differences can be directly attributed to changes in base sequence, the remaining differences were not at the base substitution sites. Some of the differences were over fifty base pairs from the nearest base substitution. We propose that the majority of these differences are due to microvariation in the structure of DNA with a slightly different DNA sequence.