We have generated seven monoclonal antibodies (mAb) that recognize the T200 molecule. These mAb have been classified by competitive binding assay in flow cytometry into three groups each reacting with a different epitope of the T200 molecule: (a) 136-4B5, that shows a sialic acid nature, (b) 135-4H9, 135-4C5, 144-2, 155-2 and (c) 72-5D3, 124-2H12b. A heterogeneous effect was observed when they were tested on an anti-immunoglobulin-induced B cell proliferation. Whereas 72-5D3 and 135-4H9 mAb inhibited the proliferative response of B cells, 136-4B5 mAb greatly enhanced it, both effects being dose dependent. We can conclude that anti-CD45 mAb have a different and contrary functional behavior on anti-Ig-induced B cell proliferation, depending on the epitope recognized. The basis for such a difference could reside in the glucidic nature of the epitope recognized by the 136-4B5 mAb.