Mannose-binding lectin (MBL) is an important complement-activating protein of the human innate immune system. Deficiency of MBL is associated with an increased risk of various infections and arises from three structural gene mutations in exon 1 (variants B, C and D) and/or the presence of a low efficiency promoter. The C allele is found in sub-Saharan Africa whereas the B allele is found elsewhere, suggesting that these mutations occurred after the suggested hominid migration out of Africa [100-150 000 years before present (BP)]. Paradoxically, these alleles may have a selective advantage in protection against intracellular pathogens and occur at particularly high frequencies in sub-Saharan Africa (C variant) and South America (B variant). Since hominids reached Australia at least 50 000 years ago, a study of MBL polymorphisms in the indigenous population was of interest. Using heteroduplex technology we found a paucity of MBL structural gene mutations in two population groups from geographically distinct regions. Of 293 individuals tested, 289 were wild-type and four were heterozygous for either the B or D allele. In each individual with an MBL mutation the HLA haplotype profile suggested some Caucasian admixture. We also found a restricted range of MBL promoter haplotypes and the serum MBL levels were higher than those of any other ethnic group studied to date (median 3.07 microg/ml). Our data suggest that the B mutation probably arose between 50 000 and 20 000 BP. Its absence from the founder gene pool of indigenous Australians may also partly explain their vulnerability to intracellular infections such as tuberculosis.