The ligand-binding domain of the chicken type-I interleukin-1 (IL-1) receptor (soluble IL-1R(I); sIL-1R(I)) was cloned into a Pichia pastoris expression system and the resulting sIL-1R(I) binding protein was used to produce antisera in rabbits (anti-IL-1R(I)). Two experiments were conducted to determine the capacity of sIL-1R(I) or anti-IL-1R(I) to block the IL-1 bioactivity (thymocyte co-stimulation) in conditioned media (CM) from HD11 chicken macrophages stimulated with lipopolysaccharide. In the first experiment, pre-incubation of CM with unpurified sIL-1R(I) significantly decreased its thymocyte co-stimulation activity by 57%. Further purification of sIL-1R(I) from other proteins secreted or shed from P. pastoris expression system by size exclusion filtration or ammonium sulfate (60%) precipitation did not influence its capacity to neutralize IL-1 bioactivity. These partially purified sIL-1R(I) preparations significantly decreased thymocyte co-stimulation activity in CM by 70.7 and 77.3%, respectively. In the second experiment, pre-incubation of thymocytes with antisera against the sIL-1R(I) decreased IL-1 activity in CM by 70% relative to control thymocyte cultures that received no antibody and by 59% relative to thymocyte cultures incubated with pre-immune sera. Presumably anti-sIL-1R(I) diminished the IL-1 bioactivity in CM by blocking IL-1 binding to its type-I receptor on thymocytes. Thus, 30% of the IL-1-like activity released by LPS-stimulated HD11 macrophages is probably due to at least one other cytokine. Our data are consistent with the type-I receptor being the primary IL-1 receptor on chicken thymocytes that is capable of providing a signal for proliferation.