The use of a white-ivory (w(i)) strain of Drosophila melanogaster carrying four copies of this allele, (w(i))4, has proved to be useful in detecting somatic mutation in genotoxicity testing. Nevertheless, until now very little information exists about the nature of the genetic effects detected in such a strain. This work presents molecular data on the changes that have taken place in different germinal mutants obtained after treatment with alkylating agents. Three different phenotypes were obtained: wild-type red eyes, dark red eyes and eyes lighter than (w(i))4. Our results show that, in at least one of the four copies of the allele, the wild-type red eye phenotypes are due to a precise excision of the 2.96 kb duplicated region characteristic of the w(i) allele. These data agree with previous results obtained in a strain carrying only a single copy of the w(i) allele. The dark red eye mutants analysed seemed to be generated as a cluster and all proved to be caused by deletions at the 3'-end of the duplicated w(i) region in two of the copies of the (w(i))4 genome. Finally, the light eye mutants (obtained at high frequencies) failed to show alterations at the molecular level, although we cannot discard the possibility that they might have originated by the loss of some of the w(i) copies of the (w(i))4 strain.