IL-1 beta is a stimulator of hematopoietic and inflammatory systems, and also acts as a radioprotector. After whole-body exposure to sublethal doses of ionizing radiation, the IL-1 beta mRNA level in spleen cells increases for a short time prior to regeneration of the spleen. We analyzed spleen cells of C3H/He mice after whole-body irradiation with 3 Gy x-rays to determine the cause of this short-term increase in the transcription level. An increase in the level of the message in spleen cells, found by Northern blot hybridization, reached its peak 5 to 7 days after irradiation. There was a low correlation between the curves of the mRNA level and the ratio of monocyte/macrophage lineage cells; a typical source of the message. Spleen macrophages that produce a large amount of the message were found 7 days after irradiation in an in situ hybridization experiment in which heterogeneous spleen cell populations were used. In contrast, spleen cells had no detectable levels of macrophages rich in IL-1 beta mRNA before and 17 days after irradiation. Additionally, the population of message-rich cells was 9.4% of the total number of monocytes/macrophages in the spleen. These results suggest that the short-term increase in IL-1 beta mRNA is a result of the heterogeneous differentiation of a subpopulation of spleen macrophages before regeneration of the spleen.