Mouse milk cells were stained with rhodamine or fluorescein isothiocyanate and fed to young suckling mice. By visual examination of serial sections and by flow cytofluorometry, we were able to demonstrate directly the presence of these cells in peripheral tissues. It was estimated that at least 0.1% of the fed cells might infiltrate the young mouse, which is initially immunologically defenseless. This is in accordance with evidence from many sources for activity of maternally-derived lymphoid cells in young rodents.