Genetic markers were analysed in 48 adults who appeared to have a prolonged infection with Giardia lamblia. The frequency of ABO blood groups, Rhesus blood groups, and Gm phenotypes was similar to that in control subjects. However, there was a higher than expected frequency of HLA antigens A1 (observed 46 . 7%, expected 32%) and B12 (observed 47 . 8%, expected 25 . 8%) and a higher than expected frequency of the phenotypes A1/A2 and B12/B27. Genetic studies were also performed with inbred strains of mice showing relatively rapid (BALB/c) and defective (C3H/He) spontaneous elimination of Giardia muris. From analysis in backcross mice several genes appeared to influence susceptibility to prolonged infection with G. muris.