We have searched for dominant modifiers, i.e., enhancers and suppressors, of the compound eye phenotype of split, a recessive viable allele of Notch. Among the spl modifiers found, we have detected mutations in loci whose functions were previously known to cooperate with Notch in embryonic neurogenesis, such as daughterless, master mind, Delta and Hairless. In addition, other spl modifier mutations have been found in loci that were not previously known to interact with Notch, such as scabrous, glass, roughened eye, and several other genes that have not yet been assigned to known loci. The phenotypes associated with mutations in some of these latter loci suggest the participation of the corresponding genes in embryonic neurogenesis. We show that in some cases the observed interactions are due to genetic haplo-insufficent expression of the genes, whereas allele-specific interactions with spl are observed in master mind and Delta alleles. From this observation, we propose a direct functional association between the proteins encoded by Notch, Delta and master mind.