Chimaeric mice were produced by aggregating two morulae-one of homozygousrdsmutant and another from a strain of mice with normal retina, which also differed in colour genes. The interphotoreceptor matrix in the retinal sections of these chimaeras was studied histochemically. In sections, stained with colloidal iron, regions withrds/rdsphotoreceptor layer, characteristically lacking the outer segments, showed more intense staining of the interphotoreceptor matrix, while regions with normal receptor outer segments showed less intense staining of the matrix. In sections, stained with toluidine blue,rds/rdsregions showed more intense reaction along the pigment epithelial-photoreceptor interface and less intense reaction over the inner segments in comparison to the regions with normal photoreceptors. These differential staining reactions were independent of the overlying retinal pigment epithelial cell genotype and resembled the reaction patterns in the retina of pure strain controls of the same age. Small patches, showingrds/rdstype staining were also observed within areas which appeared normal.We suggest that the altered properties of the interphotoreceptor matrix in therdsretina result from gene expression within the photoreceptor cells.