The lymph node and spleen cells of mice painted on the skin with the contact-sensitizing agent, picryl chloride, transfer contact sensitivity. Their ability to transfer reaches a peak 4 days after immunization and is absent by day 6 providing the recipient mice are challenged shortly after transfer (Chase type transfer). In contrast, when challenge of the recipients is delayed for 6 days (adoptive transfer), lymph node and spleen cells show the greatest ability to transfer 8–12 days after immunization. When cells taken 4 days after immunization (which transfer contact sensitivity) are mixed with cells taken at 6–11 days (which fail to transfer), the mixture shows little ability to transfer. This provides evidence for the occurrence of suppressor cells. Lymph node and spleen, and thymus cells show suppressor activity. The suppression is specific and cells from donors immunized with the contact-sensitizing agent oxazolone will not suppress passive transfer of contact sensitivity to picryl chloride. The occurrence of suppressor cells is the main cause of the loss of ability of lymph node and spleen cells in transfer later than day 6 after immunization. Experiments on the loss of radioactivity from lymph nodes labelled with 125I-iododeoxyuridine (IUDR) suggest that loss of cells from the lymph nodes may be a contributory factor.