To evaluate the biological effects of over‐expression of interleukin‐1β (IL‐1β) on the immune system we have generated transgenic mice, expressing the IL‐1β gene fused to a heterologous signal sequence under the control of the mouse immunoglobulin enhancer (Eμ). A prominent hyperplasia and a disturbed microarchitecture of lymphoid tissues were observed in the transgenic mice. The CD4⁺ T cells in the hyperplastic lymphoid organs seemed to invade the majority of the lymphoid organs including B‐cell restricted areas. Analysis of lymph node cells revealed an increased frequency of CD4⁺ CD44^high CD62L⁻ T cells and local secretion of IL‐2 and IL‐4, compatible with an elevated number of activated T cells. Furthermore, significant levels of human IL‐1β in sera and high concentrations of serum immunoglobulin G (IgG) were observed in the transgenic mice. The data suggest a role for IL‐1β in controlling lymphoid microarchitecture and, when over‐expressed, breaking the threshold in T‐helper–B‐cell interaction.