SUMMARYA study has been made of the size, frequency and histochemical reactions of PAS‐positive reticulum cells in the thymus cortex of high‐ and low‐leukaemia strain mice.In high‐leukaemia strain thymus glands there were three times as many mitoses in lymphocytes surrounding these cells as in lymphocytes elsewhere in the thymus. The incidence of early stages of mitosis was also higher in PAS cell‐associated mitoses than in mitoses elsewhere in the thymus. In low‐leukaemia strain thymus glands the association of PAS cell and lymphocyte mitoses was less evident.The development of PAS cells in thymus grafts was related to the age of the thymus graft and not affected by circulating factors in the host animal.