The β-adrenoceptor agonist, isoprenaline, inhibited the immunoglobulin E-mediated release of histamine from human lung mast cells (HLMC). Long-term (24 h) exposure of HLMC to isoprenaline reduced the subsequent effectiveness of isoprenaline to inhibit histamine release. The extent of this functional desensitization was variable with some HLMC preparations resistant and others highly susceptible. We sought to determine whether the variability in the degree of functional desensitization was influenced by genetic polymorphisms in the β2-adrenoceptor. HLMC preparations were genotyped at two polymorphic loci, positions 16 (arg to gly) and 27 (gln to glu), and the effect of desensitizing conditions (24 h with 10−6m isoprenaline) on the subsequent ability of isoprenaline (10−7m) to inhibit histamine release from HLMC was determined (n= 72). In HLMC preparations expressing β2-adrenoceptors with arg (wild-type) or gly (mutant) at position 16, desensitization was 71 ± 5% (n= 18) or 43 ± 5% (n= 26), respectively, whereas the desensitization was 59 ± 6% (n= 28) for heterozygotes at this position. In HLMC preparations expressing β2-adrenoceptors with gln (wild-type) or glu (mutant) at position 27, desensitization was 65 ± 5% (n= 25) or 28 ± 7% (n= 17), respectively, whereas the desensitization was 61 ± 5% (n= 30) for heterozygotes at this position. These data suggest that mutant (gly16and glu27) forms of the receptor are resistant to desensitization compared to wild-type (arg16and gln27) forms. However, analyses to determine the relative contributions of positions 16 and 27 suggest that position 27 is more important in influencing the degree of functional desensitization.