As result of the First International Swine CD Workshop, six monoclonal antibodies (mAbs) (numbers 014, 023, 024, 057, 128, and 130) clustered closely to the internal standard anti-porcine CD2 mAb, MSA4. Despite the close clustering, the cluster was split into two subgroups. To further characterize the relationship between these mAbs, they were used in flow cytometry to inhibit binding of MSA4 to porcine lymphocytes. mAbs 014 (1038-8-31), 023 (MAC83), 024 (MAC80), 057 (PG168), and 128 (MSA4) completely inhibited the binding of MSA4, mAb 130 (MSA2) failed to inhibit MSA4 binding. On dual parameter flow cytometry comparing MSA2 with MSA4, all MSA2(+) cells were MSA4(+), two thirds of the MSA4(+) cells were MSA2(-). We conclude that five of the mAbs bind to the same or a closely related epitope on porcine lymphocytes. mAb 130 appears to have aberrantly clustered with the CD2 group of mAb.