Interphotoreceptor retinoid-binding protein (IRBP) was studied using immunochemical and immunocytochemical techniques in retinae of mice with allelic combinations at the rd and rds loci at different stages of development and degeneration. Until postnatal day 7 (P7), IRBP is located intracellularly in developing retinae of the different genotypes. Thereafter, IRBP is present mainly in the interphotoreceptor matrix. As previously noted, cell death is slowest in the heterozygous +/+,rds/+ mutant with loss increasing in order in +/+,rds/rds, rd/rd, rds/rds and rd/rd,+/+ animals. The IRBP content of the total retina also approximates this pattern, with lowest amounts by far in rd/rd, rds/rds and rd/rd,+/+ mutants (after P14). Interestingly though, IRBP loss significantly precedes visual cell loss in the rd/rd,rds/rds retina. In all the mutants, the remaining rod cells in the outer nuclear layer exhibit synthesis of intracellularly located IRBP at late stages of degeneration. In the single homozygous rd/rd,+/+ and the double homozygous rd/rd,rds/rds mutants, IRBP is present intracellularly during the entire degenerative process with somewhat less intracellular IRBP in the rd/rd,rds/rds mutant. Retinae of homozygous +/+,rds/rds and heterozygous +/+,rds/+ animals exhibit a normal distribution pattern of IRBP immunoreactivity until loss of photoreceptor cells becomes pronounced at later stages of the disease. Many of the remaining cells at this time are probably cone elements although they are structurally changed. Double labeling with IRBP and S-antigen demonstrates, in many but not all, the presence of both proteins in the same cell body. Immunocytochemistry clearly demonstrated the presence of IRBP in remaining photoreceptor cells at late stages of the disease. Thus, the biochemically measured loss of IRBP appears to be a complex process neither directly dependent on the loss of photoreceptor outer segments and reduced interphotoreceptor matrix space (e.g. there is a sustained IRBP level in rodless rds mutants) nor simply due to cell death (e.g. in the rd/rd,rds/rds mutant, IRBP loss significantly precedes cell loss). That this IRBP is mainly intracellular, however, may indicate an abnormality in secretion which, combined with other factors, induces a degenerated and less differentiated phenotype.