In situ hybridization was used to examine the expression of c-fos mRNA in mouse thymus, spleen and lymph node following whole body irradiation(WBI) with 75mGy X-rays. The results showed that the basal expression of c-fos mRNA in macrophages, interdigitating cells and some large lymphocytes of thymus, spleen and mesenteric lymph node was markedly enhanced after WBI with 75mGy X-rays. The expression of c-fos mRNA in the thymus and spleen reached its peak 1 h and 2 h after irradiation respectively and both returned to the sham-irradiated level 12 h after irradiation. However, the increase in expression of c-fos mRNA in the lymph node was not so marked but lingered longer after irradiation. The implications of these findings were briefly discussed in connection with our previous data.