Pigs were skin painted with the contact sensitizing agent DNFB and afferent lymph was collected for the next 24 h. The lymph cells carried a small amount of DNP and were able to sensitize 40% of homologous recipients. Peripheral blood lymphocytes conjugated with DNFB in vitro to the same degree as afferent lymph cells sensitized 80% of autologous and 40% of homologous recipients: lymphocytes conjugated at higher levels sensitized 75% of animals in each group. Lightly conjugated cells were capable of survival and able to respond to stimulation by mitogens and their ability to sensitize autologous recipients was abolished by heat killing. Highly conjugated cells were not capable of survival, their sensitizing ability was not altered by heat killing and they were able to sensitize incompatible recipients. It is suggested that highly conjugated cells sensitize by a different mechanism which depends on the cooperation of non-lymphocytic cells, not easily mobilized from lymphoid tissue.