In contrast to the uniform rejection of adult male skin isografts by C57BL/6 females, neonatal male skin isografts are frequently accepted. Moreover, 50% of all females which accept a neonatal male skin graft for 50 days accept a subsequent adult male skin graft as well. This ability of neonatal skin to produce tolerance has been investigated under a variety of experimental conditions. The results indicate: (a) Even when a newborn male skin graft is transplanted concomitantly with an adult graft, it can produce tolerance of the latter although it is less effective in this regard than when transplanted beforehand. (b) The continued exposure of the host to the newborn graft is vitally important in maintaining the unresponsive state; and most females deprived of these grafts for 50 days manifest an immune response when challenged with adult male skin. (c) Newborn male skin isografts raised on adult females are not as antigenic as normal male skin grafts. (d) Occasionally, even a presensitized female can be rendered tolerant by grafting with neonatal male skin. (e) Neonatal male skin grafts are not accepted when transplanted to the spleens of adult females although they may occasionally induce tolerance of a subsequent orthotopic adult male skin graft. The failure of these intrasplenic grafts to survive can be attributed at least partly to their small size since orthotopic grafts of comparable size usually do not survive. (f) Females bearing neonatal male skin grafts are not perceptible cellular chimeras. Because the unresponsive condition induced with neonatal skin is similar to that which results from multiparity, this latter condition has also received attention. In this regard it has been established that unlike the removal of a neonatal male skin isograft, the delayed grafting of isolated females with a previous history of multiparity does not result in many of them manifesting what may be considered an immune response. However, this delay in grafting does seem to impair the tolerance multiparity produces. The results are discussed in relation to other methods of producing tolerance in adult animals.