[en] In these studies the authors examined the effects of X-rays and of formaldehyde on the excision of a mobile DNA sequence (P-element) from the genome of Drosophila, in the presence and absence of an MR chromosome, a second chromosome carrying 2 complete P-factors. The mutator activity of MR chromosomes is largely due to the transposition of these P-elements. The excision can be measured by the reversion of a mutation, causing singed bristles (resulting from a P-element insertion) to wild-type. The results indicate that (1) X-rays affect excision of the element directly, in the absence of MR whereas no effect was observed in the presence of MR, and (2) formaldehyde does not directly affect the excision, but is effective in decreasing the P-transposon-mediated excision. Concurrent experiments on sex-linked recessive lethals with or without MR and X-rays or formaldehyde were carried out. These data showed that the effect of formaldehyde on reversion of sn is small in comparison to its mutagenic effectiveness, as measured by the induction of recessive lethals. (Auth.)