Glutaredoxins (Grx) catalyze glutathione-dependent thiol-disulfide oxidoreduction reactions. Mammalian cells contain at least two dithiol glutaredoxins, the well-characterized cytoplasmic (12kDa) Grx1 and the recently identified (18kDa) Grx2 with mitochondrial and nuclear isoforms. We have developed two sensitive and specific sandwich ELISAs to study the levels of human Grx1 and Grx2. Both Grx1 and Grx2 were present in placenta extracts and in cell lysates prepared from various tumor cell lines. However, the levels of Grx1 were at least 20 times higher than those of Grx2. Plasma from healthy blood donors contained 13.4+/-7.9ng/ml of Grx1, while Grx2 was not detected. Unstimulated peripheral blood mononuclear cells were shown to secrete Grx1, but upon 12-O-tetradecanoylphorbol-13-acetate activation, the secretion of Grx1 was strongly suppressed. This effect was shown to occur at the transcriptional level. The secretion of Grx1 and its presence in plasma suggests extracellular functions as found for mammalian thioredoxin 1.