Interleukin-1 (IL-1) has been shown to stimulate hematopoietic progenitor cell growth both in vitro and in vivo. Although IL-1 alone lacks the ability to promote hematopoietic progenitor growth in vitro, it is a potent synergistic factor in combination with other colony- stimulating factors (CSFs). Because it was unknown whether type I (p80), type II (p68), or other IL-1-binding proteins mediated the synergistic effects of IL-1 on purified progenitor cells, we used the difference in immunoreactivity between type I and type II IL-1 receptor (IL-1R) to better assess the role of these receptors in hematopoietic progenitor growth. Therefore, the synergistic effects of IL-1 alpha on IL-3-, CSF-1-, and granulocyte macrophage (GM)-CSF-induced progenitor growth, both in CFU-c and single-cell assays, were determined in the presence of monoclonal antibodies (MoAbs) 35F5 and 4E2 that block the binding of IL-1 alpha to type I and type II IL-1R, respectively. The synergistic effect of IL-1 alpha on IL-3 responsive Lin- and Lin(-)-Thy- 1+ progenitors was indirectly mediated and could be inhibited by MoAb 35F5. In contrast, IL-1 alpha directly synergized with CSF-1 and GM-CSF to promote progenitor cell growth. The direct synergistic effect of IL- 1 alpha on CSF-1-induced progenitor growth was observed in all progenitor populations examined (Lin-, Lin-Thy-1+, and Lin-Thy-1-) and was inhibited by MoAb 35F5. However, the direct synergistic effect of IL-1 alpha on GM-CSF-responsive progenitors. Lin- and Lin-Thy-1+, was partially inhibited by MoAb 35F5. In contrast, the MoAb antitype II IL- 1R (MoAb 4E2) could not inhibit the direct synergistic effects of IL-1 alpha on CSF-1- or GM-CSF-induced progenitor growth. Thus, IL-1 alpha directly and indirectly stimulates the growth and differentiation of purified progenitors through the type I IL-1R but not the type II IL-1R.