CTLs at 13-16 days after stimulation were labelled overnight in the presence of 1-2mg/ml HRP added directly to the growth medium to load the lytic granules then washed 3 times by pelleting and resuspending in RPMI to remove free HRP and serum and the final pellet resuspended at 5x10^6 cells/ml. CTL were mixed 1:1 with targets. 1mug/ml PHA or anti-CD3 (UCHT-1) was added for conjugation of human CTL. Conjugates were left in suspension at RT for 5 min then plated in individual wells of 4-well plastic tissue culture plates (Nunc) and transferred to 37¿C for a further 30-60 min. Samples were fixed and processed for DAB-cytochemistry, post-fixed with reduced osmium and EPON embedded. semi-thick were stained with lead citrate and coated with 10nm fiducial gold markers for tomograhpic reconstruction.