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The classic histamine release test involved washing and isolating of leukocytes from heparinized whole blood. Siraganian automatized the extraction procedure, working initially with isolated leukocytes in Tris-human serum albumin-Ca++Mg++ buffer, with optimal results. One year later, he described a continuous flow system that permitted the study of whole blood from atopic patients. This method was easier to perform since the isolation of the white blood cells is no longer necessary. On the other hand, the results working with whole blood seem to be better than with isolated leukocytes. We have published modifications of the continuous flow technique, working also with whole blood. Our results agree with those of Siraganian and others. An alternative to working with whole blood or isolated cells in synthetic buffers could be the use of autologous diluted plasma. In this way, the cells are centrifuged and the plasma poured off. The plasma can be used for other determinations, such as RIA, ELISA, etc. The blood is reconstituted to its original volume with a PBS-Ca++Mg++ buffer and processed as described. The results using the whole blood and "washed cells" were very similar. But in some cases we found positive histamine release in "washed leukocytes" but not in whole blood. This striking phenomenon can be explained by the presence in plasma of blocking non-IgE antibodies. If this is the case, the removal of approximately 85% of the plasma will also lead to the disappearance of the 85% of these blocking antibodies. On the other hand, all the positive results obtained using whole blood have been confirmed using "washed cells". Therefore, the use of "washed cells" has several advantages with respect to whole blood assay or leukocyte isolation.