Neonatal skin grafts across a weak histocompatibility barrier (MSA-incompatible) were used only for sensitization of the recipients whose responsiveness was then tested in two different assays. On the basis of 1. functional inactivation of PEC in a PEC transfer system and 2. inhibition of macrophage migration. The difference between neonatal and adult skin grafts turned out to be quantitative rather than qualitative, i.e., both induced sensitization whose demonstration with the neonatal grafts required more sensitive techniques and/or more favourable timing because it was weaker. The possible nature of the difference, which was occasionally interpreted as being due to a tolerogenic rather than immunogenic activity of neonatal grafts, is discussed.