APO-1/Fas (CD95) is a transmembrane receptor that transduces apoptotic signals within various cells including T and B cells. The APO-1 gene was found to be defective in lpr mice. In these mice insertion of a retrotransposon gives rise to transcription of an abnormal mRNA and only a fraction of wild-type APO-1 mRNA. It is not clear if lpr mice still express wild-type APO-1 protein. To address this question, we prepared rabbit anti-APO-1 antibodies (Ab) with a peptide representing the extracellular sequence corresponding to residues 5-23 of APO-1. The rabbit Ab reacted with thymocytes from different mouse strains and the extent of binding was correlated with the two known APO-1 alleles. In addition, the Ab reacted with mouse cell lines expressing mouse APO-1 mRNA but not with human cell lines. Binding of the Ab to MRL and BALB/c thymocytes was completely blocked by the immunizing peptide. Immunofluorescence analysis of MRL/lpr thymocytes showed that they still express APO-1 protein at approximately one tenth of the wild-type expression level on their surface. In addition, in lpr as in wild-type mice we found a decrease of APO-1 expression in the more mature thymic compartment. Western blot analysis of whole cell lysates from lpr and wild-type thymocytes showed that the Ab recognized APO-1 in both cell types. Approximately 50% of CD3+ splenocytes and 80% of in vitro activated CD3+ cells from wild-type mice reacted with the Ab, but to a lower extent than thymocytes. The same differential reactivity was found in lpr CD3+ splenocytes. lpr T cells, however, showed a substantially lower level of APO-1. Thus, the differential expression of APO-1 on thymic versus peripheral lpr T cells might influence their sensitivity towards APO-1-mediated apoptosis.