Abstract A cDNA clone of prostaglandin F synthase (PGFS) was isolated from human lung by using cDNA of bovine lung-type PGFS as a probe and its protein expressed in Escherichia coli was purified to apparent homogeneity. The human PGFS catalyzed the reduction of prostaglandin (PG) D 2, PGH 2 and phenanthrenequinone (PQ), and the oxidation of 9α,11β-PGF 2 to PGD 2. The k cat/ K m values for PGD 2 and 9α,11β-PGF 2 were 21 000 and 1800 min −1 mM −1, respectively, indicating that the catalytic efficiency for PGD 2 and 9α,11β-PGF 2 was the highest among the various substrates, except for PQ. The PGFS activity in the cytosol of human lung was completely absorbed with anti-human PGFS antiserum. Moreover, mRNA of PGFS was expressed in peripheral blood lymphocytes and the expression in lymphocytes was markedly suppressed by the T cell mitogen concanavalin A. These results support the notion that human PGFS plays an important role in the pathogenesis of allergic diseases such as asthma.