Possible factors responsible for the initiation of mitotic activity in “gradient” cultures of leukocytes from normal human blood were investigated. Variations of temperature, pH, oxygen tension, carbon dioxide tension, plasma and cell concentrations, as well as the amount of agitation, over at least as wide a range as might be encountered in vivo, produced only moderate quantitative changes in mitotic activity.

The mucoprotein plant extract, phytohemagglutinin (PHA), employed originally as a means of separating the leukocytes from whole blood in preparing the cultures, was found to be a specific initiator of mitotic activity: in its presence, cell division occurred; in its absence, no mitoses appeared. The studies suggest that the mitogenic action of PHA does not involve mitosis per se but rather the stage preceding mitosis—the alteration of circulating monocytes and large lymphocytes to a state wherein they are capable of division. The relationship of this mitogenic action of PHA to mitotic and premitotic processes in the body remains to be investigated.