In the complement dependent lymphocytotoxic microtechnique it was found that antibody-killed frozen B lymphocytes are sufficiently stained for reliable reading after 30 min of incubation with trypan blue, while the background of staining is only about 10%. During the next 30 min of incubation, however, the background of staining increases to about 30%, whereafter it remains constant for at least 24 h. Formaldehyde is able to stop the trypan blue uptake by killed or damaged lymphocytes completely. Consequently, if formaldehyde is added to the reactions 30 min after the trypan blue addition, the otherwise rapidly increasing background of staining is kept at an acceptable level of 10%, thus making HLA-DR typing of frozen stored B lymphocytes possible. The trypan blue staining seems rather independent of incubation conditions before the addition of the dye. Similar results were obtained with T lymphocytes.