Abstract
1. Various concentrations of PHA were utilized to determine the optimal subthreshold concentration of PHA for the maintenance of lymphocytes in vitro. Although 0.1, 0.3 and 0.6 ml. of PHA (diluted one hundred-fold) appeared to be similar in effectiveness during the initial 35 days of culture, 0.1 or 0.3 ml. of PHA were more effective over longer periods of time.
2. Whether the culture medium was changed at weekly intervals or not had little effect on the culture during the initial 35 days. Over the long run, however, changing the culture medium every week resulted in a larger number of surviving lymphocytes.
3. Many phagocytic cells could be observed during the entire period of cultare. Most conspicuous were clumps of cells consisting of a central macrophage surrounded by blasts and a few small and medium-sized lymphocytes.
