Among 54 mAb determined to be reactive with porcine T lymphocytes and/or activation antigens, eight mAb (workshop Nos. 005, 031, 080, 091, 092, 093, 094 and 110) derived from different laboratories grouped together in the T11 cluster and were ordered into the SWC1. One mAb (No. 111) which belong also to this group was lost during the workshop. The SWC1 antigen is a molecule expressed on the majority of leukocytes, resting T lymphocytes, monocytes and granulocytes, but not on B lymphocytes. On T Iymphocytes it is down-regulated after activation. The molecular mass of the antigen is unknown. Epitope analyses revealed that seven out of the nine mAb recognized similar epitopes on the SWC1 molecule.