We have reported on the expression and characteristics of IL-13R and have demonstrated that IL-13 competes for IL-4 binding while IL-4 did not compete for the IL-13 binding on some cell types. Based on these observations, and the size of IL-13 and IL-4 cross-linked proteins, we concluded that the receptor for IL-13 is complex and shares a subunit with the receptor for IL-4. To explore the complexity of the IL-13R, a wide variety of cell types was examined for IL-13 and IL-4 binding. We report in this work that IL-4 does not always bind well to cells that bind IL-13, but the reverse is also true. We also found that IL-4 can compete more effectively for IL-13 binding than IL-13 itself. Cross-linking studies support these observations and demonstrate that 125I-labeled IL-13 bound exclusively to a single 65- to 70-kDa protein in MA-RCC and U251 cells, while in TF-1 cells it cross-linked to two membrane proteins of 65 to 70 kDa and 140 kDa. Furthermore, by using a chimeric protein composed of IL-13 and Pseudomonas exotoxin A, we observed that IL-4 neutralized the cytotoxicity of the IL-13 toxin on COS-7 cells by blocking a common form of the two cytokine receptors. We propose that the 65- to 70-kDa form of the IL-13R is the predominant common component shared between IL-13 and IL-4R. However, the primary IL-4 binding (p140) protein also participates in the formation of the IL-13R complex in some cell types. In addition, the gamma(c) or another interactive subunit may influence IL-13 binding to its receptor complex. Thus, we propose that there are at least four forms of IL-13R.