In the present study, we have demonstrated platelet-type 12-lipoxygenase (12-LOX) expression in human rheumatoid arthritis (RA) type B synoviocytes by reverse-transcription polymerase chain reaction (RT-PCR). The presence of 12-LOX mRNA in these cells was revealed by classical RT-PCR analysis using platelet-type 12-LOX cDNA primers and the PCR fragment (246 bp) was purified, amplified and sequenced. By sequence analysis, this fragment was determined to be 100% identical to that from platelet-type 12-LOX cDNA. Immunofluorescence data demonstrate that interleukin-1beta (IL-1beta) increases cellular 12-LOX protein. Other results associate specific inflammatory cytokines with the activity of 12-LOX in human RA type B synoviocytes. IL-1beta increased 12S-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) production (4-fold) and we also observed an increase in 12-HETE production (2.5-fold) after incubation of human RA type B synoviocytes with TNF alpha. In contrast to the action of IL-1beta on 12-HETE synthesis, IL-4 and IL-6 did not enhance 12-HETE production. This is the first demonstration of platelet-type 12-LOX cDNA derived from the mRNA of cultured human RA type B synoviocytes.