The in vivo clastogenicity of 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide (AF-2) was examined in the micronucleus test using peripheral blood from three mouse strains (ICR, CD-1, and MS/Ae) and bone marrow from one rat strain (Sprague-Dawley). Doses up to the maximum tolerated were tested. The chemical was given once, twice, thrice, or four times via either the i.p. or p.o. route. Under some conditions, ICR and CD-1 mice showed an increased frequency of micronucleated reticulocytes, but definite conclusions were difficult to draw because the increases were very slight. MS/Ae mice showed a markedly elevated micronucleated reticulocyte frequency after the double and triple ip treatments. Rats showed a slightly but statistically significantly increased frequency of micronucleated polychromatic erythrocytes after double i.p. treatments. These results indicate that AF-2 is a weak in vivo clastogen.