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CD27 is a member of the nerve growth factor receptor family, that includes two types of tumor necrosis factor receptor, CD40 and Fas/Apo‐1. Human CD27 has been found only on lymphocytes. In T cells, its expression strongly increases in a transient fashion upon antigenic stimulation, suggesting that CD27 plays a role during T cell activation. To analyze the function of CD27, we have identified the murine CD27 at the cDNA and protein level. Murine CD27 shows an identity of 65% compared with human CD27. The amino‐terminal cysteine‐rich region, i.e. the putative ligand‐binding domain, and the carboxy‐terminal part of the cytoplasmic domain are approximately 80% identical in man and mouse. Murine CD27 has 29% identity to 4‐IBB, another lymphocyte‐specific member of the receptor family defined only at the cDNA level. Murine CD27 and 4‐1BB have 39% homology in the cysteine‐rich domain and share a conserved region in the cytoplasmic tail. Expression studies identified murine CD27 mRNA in thymus and spleen, but not in non‐lymphoid tissues, while 4‐IBB mRNA was not detected in any tissue tested. In resting T cells, only murine CD27 mRNA was found, while in activated T cells murine CD27 as well as 4‐1BB were present at high levels. Murine CD27 and 4‐1BB mRNA are expressed with different kinetics during T cell activation, suggesting that these molecules play different roles in this process. Peptide antisera identified murine CD27 as a 45‐kDa protein on thymocytes and activated T cells, while 4‐1BB was precipitated as a 35‐40‐kDa protein from activated T cells.