Two distinct receptors for tumor necrosis factor (TNF) of 55 and 75 kd are expressed at low levels by various cells. The 55 kd TNF receptor was purified from HL60 cells, and partial amino acid sequences were determined. Short degenerate sense and antisense oligonucleotide primers encoding the N- and C-terminal ends of a peptide of 22 amino acid residues were used to amplify a 66 bp cDNA fragment from HL60 RNA by reverse transcriptase-polymerase chain reaction. The cDNA fragment as a probe identified several overlapping clones in a human placenta cDNA library. The open reading frame of the cDNA predicts a 455 amino acid TNF receptor protein with leader, extracellular, transmembrane, and intracellular domains. When expressed in COS-1 cells or in a baculovirus system, the cDNA conferred TNF binding properties comparable to the native receptor. Surprisingly, the 55 kd TNF receptor shows a high degree of sequence homology to the NGF receptor extracellular domain.