The following summary represents a consensus of the working group except where noted. The items discussed are listed in the order in which they appear in the OECD guideline (474) for easy reference. Introduction, purpose, scope, relevance, application and limits of test. The analysis of immature erythrocytes in either bone marrow or peripheral blood is equally acceptable for those species in which the spleen does not remove micronucleated erythrocytes. In the mouse, mature erythrocytes are also an acceptable cell population for micronucleus analysis when the exposure duration exceeds 4 weeks. Test substances. Organic solvents such as DMSO are not recommended. Freshly prepared solutions or suspensions should be used unless stability data demonstrate the acceptability of storage. Vegetable oils are acceptable as solvents or vehicles. Suspension of the test chemicals is acceptable for p.o. or i.p. administration but not for i.v. injection. The use of any unusual solvent should be justified. Selection of species. Any commonly used laboratory rodent species is acceptable. There is no strain preference. Number and sex. The size of experiment (i.e., number of cells per animal, number of animals per group) should be finalized based on statistical considerations. Although a consensus was not achieved, operationally it was agreed that 2000 cells per animal and four animals per group was a minimum requirement. In general, the available database suggests that the use of one gender is adequate for screening. However, if there is evidence indicating a significant difference in the toxicity between male and female, then both sexes should be used. Treatment schedule. No unique treatment schedule can be recommended. Results from extended dose regimens are acceptable as long as positive. For negative studies, toxicity should be demonstrated or the limit dose should be used, and dosing continued until sampling. Dose levels. At least three dose levels separated by a factor between 2 and square root of 10 should be used. The highest dose tested should be the maximum tolerated dose based on mortality, bone marrow cell toxicity, or clinical symptoms of toxicity. The limit dose is 2 g/kg/day for treatment periods of 14 days or less and 1 g/kg/day for treatment periods greater than 14 days. A single dose level (the limit dose) is acceptable if there is no evidence of toxicity. Controls. Concurrent solvent (vehicle) controls should be included at all sampling times. A pretreatment sample, however, may also be acceptable only in the short treatment period peripheral blood studies. A concurrent positive control group should be included for each experiment.(ABSTRACT TRUNCATED AT 400 WORDS)