Changes in the immune competence and levels of suppressore elements were assessed by mitogen stimulation and in vitro antibody production, after resection of a transplantable sarcoma. Spleen cells from tumour-resected animals were found to have depressed responses to conA as well as to the antigens SRBC and DNP-LPS. This inability to respond was gradually overcome and, by Day 21 after resection, spleen cell competence had returned to normal levels. Suppressor cells isolated from the spleens of tumour-resected animals were capable of suppressing the conA response and PFC response of normal syngeneic spleen cells in vitro. The ability to suppress the conA response of normal cells disappeared by Day 1 after resection, while the ability to suppress the anti-SRBC and anti-DNP PFC response of normal cells disappeared by Day 8 and Day 14 respectively. Serum from tumour-resected mice was also found to be suppressive to the conA response of normal spleen cells. The inhibitory material responsible for suppression eluted with the Ig-containing fraction on Sephadex G-150. This inhibitory material gradually disappeared from the serum of tumour-resected mice and was no longer apparent by Day 14. Therefore, it appeared that the return of normal lymphocyte function after tumour-resection was concomitant with the disappearance of splenic suppressor cells and suppressive serum factor.