CLEFFMANN (1963) reported that pigment cells of the lethal yellow (A-y/a) genotype change to black immediately upon cultivation, and continue to produce black pigment unless sulfhydryl reagents are added to the culture medium. We have attempted to repeat this observation and have not been able to do so. We also have been unable to indue the synthesis of yellow pigment by adding glutathione to cultures of agouti (A/A) skin. We therefore suggest that hypotheses which attempt to explain the action of agouti locus genes on the basis of effects of sulfhydryl compounds be considered with caution.