Current studies of human T lymphocytes and their subsets often use quantitative immunofluorescence analysis with monoclonal antibodies against cell surface antigens. With storage of whole blood or separated mononuclear cells for more than a few hours we have found marked changes in lymphocyte analysis using a fluorescence activated cell sorter (FACS). Experiments were done to determine if these lymphocyte changes were influenced by storage temperature and if lymphocytes could be made more stable by addition of culture media RPMI 1640 to whole blood. Optimal conditions found for blood storage were with with addition of 50% RPMI 1640 and with samples held at room temperature (22° C). With these storage conditions, delay on FACS analysis up to 24 hours did not result in spurious results. When blood samples are collected in places remote from the laboratory or when batch analysis of serially collected samples is desirable, excessive storage times should be avoided.