The response of mutagenicity to the stepwise replacement of chlorine atoms and the hydroxyl group by hydrogen in 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX, 1) was determined in several assays by using Salmonella typhimurium tester strain (TA100). In all, eight MX derivatives were assayed. Several were studied together in at least one assay. In addition to MX, the seven included 3-chloro-4-(dichloromethyl)-2(5H)-furanone (RMX, 2), 3-chloro-4-(chloromethyl)-5-hydroxy-2(5H)-furanone (3), 3-chloro-4-(chloromethyl)-2(5H)-furanone (4), 4-(chloromethyl)-5-hydroxy-2(5H)-furanone (5), 4-chloromethyl-2(5H)-furanone (6), and 4-(dichloromethyl)-2(5H)-furanone (8). Compounds 1-6 were mutagenic. Compound 8 gave erratic results. 4-(Acetoxymethyl)-2(5H)-furanone (11) was nonmutagenic. The largest drop in mutagenicity amounted to a factor of about 10(2) for the replacement of the hydroxyl group or a C-3 chlorine atom from 3. Other replacements of the hydroxyl group or a C-3 or C-6 chlorine atom amounted to mutagenicity diminished by a factor of only 10. On the basis of the rates of UV spectral changes under assay conditions, chemical half-life values (ct 1/2) for 1-6 and 8 were estimated as indicators of compound stability. However, mutagenicity differences were shown to result principally from the intrinsic mutagenicities of the six compounds 1-6 rather than from differences in stability.(ABSTRACT TRUNCATED AT 250 WORDS)