Anti-ovine IAH-CC51, anti-porcine BB6-11C9.6 and anti-human B-Ly4 mAbs are routinely used to detect porcine CD21 molecules on the surface of B lymphocytes. We have found that IAH-CC51 recognize only portion of B cells that are positive for BB6-11C9.6 or B-Ly4, which indicates probably two forms of CD21 are present in the swine, and these were assigned as CD21^a and CD21^b. While BB6-11C9.6 and B-Ly4 recognize CD21^a that is pan-specific and identify both forms, IAH-CC51 expression discriminates CD21^b+ and CD21^b— subpopulations. We have used IAH-CC51 together with anti-CD2 to define four populations of B cells in pigs: CD2^—CD21^b+, CD2⁺CD21^b+, CD2^—CD21^b— and CD2⁺CD21^b—. Further analysis of these four subset during ontogeny, expression profile in blood and MLN, their behavior in cell cultures and expression of other B cells related molecules revealed that they represent developmentally and functionally distinct subset of porcine peripheral B cells. The phenotypic and functional features suggest that CD21^b+ B cells are less mature than the CD21^b— subset. Although we were unable to isolate and characterize CD21^a and CD21^b, the results of this work are significant because (1) IAH-CC51 mAb can be used to recognize functionally distinct subsets of B cells in swine, (2) previous publications needs to be revised according to which mAb was used in which study, and (3) this is the first report showing that end-stage B cells can express differential form of CD21, which can be significant for their function. This work was supported by a grant KONTAKT ME09089 from the MSMT and grants P502/10/0038 and P502/12/0110 from the Czech Science Foundation.