THP-1 is a monocytic cell line originally derived from a patient with acute monocytic leukemia. Interactions of THP-1 cells with other cells and their microenvironment are largely determined by proteoglycans (PGs), the identity of which has not been determined. Previous studies on glycosaminoglycan expression by THP-1 cells and peripheral blood mononuclear cells from healthy individuals showed that both cell types secrete mainly chondroitin sulfate PGs to the culture medium, whereas heparan sulfate PGs are mainly retarded at the cell membrane. However, limited data on the type of PGs synthesized by THP-1 is available. In this study, the identification of PG types synthesised by THP-1 cells, which are not differentiated to macrophages, was examined. Analysis at the mRNA level by RT-PCR showed the expression of six cell membrane-associated PGs: syndecan-1,-2 and-4, glypican-1, thrombomodulin and CD44. Cell extraction, ion-exchange chromatography and dot blot analysis of the isolated PG populations with monoclonal antibodies showed the presence of syndecan-1 and thrombomodulin; the other two syndecans were not detected in any of the isolated populations. The synthesis of matrix PGs was also studied. THP-1 monocytes were positive for the mRNA encoding for versican and perlecan, but not for those encoding for decorin, biglycan, betaglycan and fibromodulin. The mRNA encoding for two versican splice variants V0 (351 bp) and V1 (386 bp), but not for V2, were identified. Biochemical analysis showed the presence of perlecan and of two populations of versican in culture medium with protein cores of average molecular sizes similar to those of V0 and V1. The production of these large matrix PGs by THP-1 monocytes is reported for the first time and may be of importance in monocyte malignant transformation and differentiation.