We recently observed that the soluble recombinant from of sheep LFA-3, termed sLFA-3 is biologically active as determined by E-rosette inhibition and inhibition of human T-cell proliferation in response to the recall antigen. In the present study, we examined the immunosuppressive properties of a derivative of sLFA-3, a dimeric form of the first domain (D1) of sLFA-3, named sD1Hcys dimer which was made by oxidative binding of the two D1 molecules through disulfide bonds formed between the SH side chains of a cysteine which was added to the C-terminal of the D1 domain. By investigating the suppressive properties of the sD1Hcys dimer, we obtained evidence that antigen-stimulated T-cell proliferation was inhibited by the suppressor T cell, mainly CD4 + CD45RA - CD45RO + and CD8 + CD45RA - CD45RO + T cells, generated by incubating PBLs with a low dose (0.5 microgram/ml) of sD1Hcys dimer in the presence of a low dose of IL-2 and GM-CSF. Flow cytometric analysis showed that the expression of some surface molecules on T cells were modulated by a high dose (5 micrograms/ml) of sD1Hcys dimer such as downregulation of CD3 and upregulation of IL-2R, but were not modulated by a low dose (0.5 microgram/ml) of the sD1Hcys dimer. These findings suggest that the sD1Hcys dimer exerts its suppressive effects on the antigen-induced proliferation assay by generating suppressor T cells. The sD1Hcys dimer might therefore have potential as an immunotherapeutic agent to inhibit and/or anergize antigen-specific T-cell responses.