Dancer heterozygotes (Dc/+) very rarely have cleft lip and show a dancing behaviour due to inner ear defects while homozygotes (Dc/Dc) have cleft lip. Males of the two genotypes Dc/+ and +/+ were mated to C3H strain and R stock females and Dc/+ males to Dc/+ females. On day 10/8 of gestation females were treated with 6-aminonicotinamide (6AN) at either 19 mg/kg or 28.5 mg/kg followed 3 h later by a protective dose of nicotinamide. Controls were untreated. Both 6AN treatments caused a significant increase in cleft lip to between 25% and 29% for crosses of Dc/+ males to C3H and R females whereas crosses with +/+ males gave 0% cleft lip. In the controls the cleft lip frequency was: for Dc/+ X Dc/+ 14%, Dc/+ X C3H 1.4%, and for the other three crosses 0%. The four crosses given the high dose of 6AN and the +/+ X C3H and Dc/+ X R cross at the low dose showed significantly increased resorption rates to between 23% and 47% over the control rates of from 5% to 11%. The presence of the Dc gene increased the susceptibility to cleft lip caused by 6AN.