Several membrane proteins prevent host cells from homologous complement attack. In humans, one such protein, decay-accelerating factor (DAF), exists as two isoforms, a GPI anchored form and a secreted form, which are generated by alternative splicing. DAF in mouse is also expressed as two isoforms, a GPI anchored form (GPI-DAF) and a transmembrane form (TM-DAF), which are produced from two separate genes. In this study, we transfected cDNA of mouse GPI-DAF or TM-DAF into Chinese hamster ovary (CHO) cells. Both isoforms of DAF on CHO cells were shown to regulate mouse complement C3 deposition mediated by the classical and alternative pathways and the inhibitory activity of both isoforms was species restricted. The two mouse DAF isoforms were effective against rat complement but not against human and guinea pig complement. Furthermore, we produced hamster mAbs to mouse DAF using GPI-DAF transfectant cells and established seven unique mAbs (RIKO-1-7). Western blotting analysis using RIKO-3, which reacts with both GPI-DAF and TM-DAF, and RIKO-4, which is an anti-GPI-DAF specific mAb, indicated that GPI-DAF was expressed on erythrocytes, spleen and testis, and that TM-DAF was expressed only in testis.