Congenic mouse strains bearing allelic variants of CD45 are often used in adoptive transfer experiments. Here, we report that immune CD8(+) splenocytes of donor origin acquired the recipient's congenic CD45 marker during interaction with antigen-bearing cells, presumably as a result of membrane transfer upon dissolution of the immunological synapse. Acquisition of recipient marker by donor cells was most prominent after in vitro incubation with peptide for intracellular cytokine staining, where most of the antigen-bearing splenocytes are of recipient origin. In consequence, when antibodies against the recipient's congenic marker were used to distinguish donor and recipient populations, donor origin cells were incorrectly interpreted as being of recipient origin. This phenomenon may cause problems for interpretation of data in adoptive transfer experiments primarily when (a) staining for the recipient's congenic marker and (b) identifying antigen-specific populations by staining for intracellular cytokine.