Patients with mutations in IL12RB1, the gene encoding IL-12Rbeta1, suffer from combined IL-12R/IL-23R deficiency and are unusually susceptible to nontuberculous mycobacteria and salmonellae. The functional effects of amino acid changes in IL-12Rbeta1, however, have not been determined at the molecular level. Molecular complementation studies are essential to demonstrate how structural amino acid changes affect IL-12Rbeta1 function, and whether functionally different IL-12Rbeta1 alleles can be distinguished. Thirteen different IL-12Rbeta1 alleles, including 11 amino acid substitutions and the two major haplotypes (214Q-365M-378G and 214R-365T-378R), were retrovirally transduced in IL-12Rbeta1 deficient human T cells. We provide functional evidence that L77P, R173P, C186S, R213W and Y367C are deleterious mutations leading to non-functional proteins. Conversely, S74R, R156H, H438Y, A525T and G594E are fully functional IL-12Rbeta1 variants. The C198R mutation leads to a partially functional IL-12Rbeta1, representing the first molecularly proven partial IL-12Rbeta1 deficiency. Interleukin-12 (IL-12) induced not only Interferon-gamma but also IL-10 in all responder but not in null-mutant alleles, with intermediate levels in C198R. The QMG allele was found to be a higher IL-12 responder allele compared with the RTR allele. These results have implications for understanding IL-12R/IL-23R structure-function and the role of IL-12R/IL-23R in human disease.