Suppressors of position-effect variegation (Su(var)s) in Drosophila melanogaster are usually studied in the presence of chromosomal rearrangements, which exhibit variegated expression of euchromatic genes moved near to, or heterochromatic genes moved away from, centromeric heterochromatin. However, the effects of Su(var) mutations on heterochromatic gene expression in the absence of a variegating re-arrangement have not yet been defined. Here we present a number of results which suggest that Su(var) gene products can interact to affect the expression of the light gene in its normal heterochromatic location. We initially observed that eye pigment was reduced in several Su(var) double mutants; the phenotype resembled that of light mutations and was more severe when only one copy of the light gene was present. This reduced pigmentation could be alleviated by a duplication for the light gene or by a reduction in the amount of cellular heterochromatin. In addition, the viability of most Su(var) double mutant combinations tested was greatly reduced in a genetic background of reduced light gene dosage, when extra heterochromatin is present. We conclude that Su(var) gene products can affect expression of the heterochromatic light gene in the absence of any chromosomal rearrangements. However, it is noteworthy that mutations in any single Su(var) gene have little effect on light expression; we observe instead that different pairings of Su(var) mutations are required to show an effect on light expression. Interestingly, we have obtained evidence that at least two of the second chromosome Su(var) mutations are gain-of-function lesions, which also suggests that there may be different modes of interaction among these genes. It may therefore be possible to use this more sensitive assay of Su(var) effects on heterochromatic genes to infer functional relationships among the products of the 50 or more known Su(var) loci.Key words: heterochromatin, chromatin, gene interactions.