ance) is an important regulatory phenomenon. After many years of investigation, it now appears that a single mechanism cannot account for all reported experiments. Among the pathways for such nega-tive immunoregulation are clone inactivation or deletion, negative feedback by antibody or antigen, active suppression by cells, and antireceptor anti-body. Even this simple list is not exhaustive since there is overlap between items, i.e., active sup-pression by cells could be mediated by excess anti-gen. It is also quite possible that more than one mechanism may be operating in a single experi-mental model. A convenient model for exploring tolerance is contact sensitivity in the mouse (Claman and Moor-
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Claman et al. (1977) studied this question.