In Drosophila, detection of chemical-induced chromosome breakage for many compounds in conventional chromosome loss or reciprocal translocation (RT) tests requires considerably higher concentrations than for sex-linked recessive lethals, or fails entirely even at concentrations giving substantial to high rates of recessive lethals. Accordingly, relatively high LDs may be necessary before evidence of chromosome breakage is observed raising questions of "false positives" resulting, for example, from aberrant metabolic effects/products not found at lower concentrations. In the test for chromosome loss, definitive scoring is made in F1 progeny. In contrast, an F2 and often an F3 is required to bring the RT test to completion. Further, the RT test is prone to "false negatives" in many cases unless testing is carried out on sperm stored in the female for several days (or longer). Accordingly, efforts in this laboratory have been directed toward improvements in the sensitivity of the test for chromosome loss. Major advances have been made by using excision repair deficient mei-9a females as P1 females to which treated males are mated. For purposes of testing, ring-X males carrying the doubly marked Y chromosome, BSYy+, were treated or not, mated for 3 days with ordinary (repair-proficient) females (the conventional test) or with mei-9a females (the mei-9a test) and all P1s discarded. Data are reported for this period only--viz, on unstored sperm. F1 progeny were scored for complete loss (CL) of the X or Y (loss is principally of the ring-X) and partial loss of the Y (PL)--viz, loss of BS or y+. Partial loss of the Y chromosome was taken as strong evidence of chromosome breakage, whereas loss of the ring-X was regarded alternatively as break-related or resulting from the production of sister chromatid exchanges. Of the 21 compounds tested, 17 are classified as carcinogens and four are unknown in this regard. At the concentration reported, 21/21 compounds were positive for CL+PL and 21/21 for PL in the mei-9a test. At the same concentration, only 10/20 compounds were positive for CL and PL and 2/21 compounds for PL with repair proficient females; the mei-9a test is clearly more sensitive than the conventional test for chromosome breakage.(ABSTRACT TRUNCATED AT 400 WORDS)