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It is shown in a large number of experiments that short ragweed extract, mixed-grass pollen extract, and pu.rified protein derivative (PPD) can produce very substantial tritiated thymidine uptake and lymphocyte blast transformation of peripheral blood leukocytes from normal individuals. Cord blood lymphocytes were also stimulated by pollen extracts. Differing conditions of culture probably explain the discrepancy between these results and many previous studies in which normal cells were not stimulated by these agents. Of particular importance are the use of human plassma in the cultures, restricted washing of the cells, the culture medium, and prolonged (8-day) cell culture times. Heparin has considerable influence on the results.

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