Other investigators have reported deficient production of interferon-gamma (IFN-gamma) and reduced expression of class II major histocompatibility (HLA-DR) antigens on monocytes from patients with retinitis pigmentosa (RP). Our previous investigation did not demonstrate deficient IFN-gamma production by lymphocytes from patients with RP. We have extended our previous study by determining the frequency of HLA-DR-positive monocytes in the peripheral blood of well-defined groups of RP patients and by including a larger sample size of patients and subdividing the autosomal dominant and recessive subpopulations. Our present results confirm and extend our previous finding that lymphocytes from patients with RP are not deficient in the production of IFN-gamma as assessed with a commercially available radioimmunoassay test kit. In addition, using two-color immunofluorescence staining and flow cytometry, we also demonstrated normal expression of HLA-DR antigens on monocytes from these patients. In both this and our previous study, using techniques employed in our laboratory, we have been unable to detect significant cell-mediated immune abnormalities in a large and well-characterized group of RP patients.