The human T-cell line Jurkat (Gillis and Watson, 1980) was cultured in RPMI 1640, supplemented with 25 mM HEPES and L-glutamine (Gibco, Tåstrup, Denmark), 10% FCS, 100 IU ml -1 penicillin and 100 mg ml -1 streptomycin. Two million cells were seeded into each well of a six-well plate and transfected with 3-4 mg of plasmid DNA (see above) and 4 ml of DMRIE-C transfection reagent (Invitrogen) according to the manufacturer's instructions. Within 48 h of transfection, the cells were washed and re-suspended at 1 ¥ 10 6 ml -1 in PBS supplemented with 2% FCS. Cells (1 ¥ 10 5 ) were incubated with human monoclonal antibodies or with a haemagglutinin mouse antibody for 30 min followed by two washes and labelling by secondary FITC-conjugated anti-human IgG or anti-mouse IgG antibody. Flow cytometry analysis was essentially as above.