Attempts have been made to confirm the findings of Metcalf. The test depends on inducing a lymphocytosis in newborn mice. For approximately 8 months, tests worked properly; and during this time, extracts of L-5178Y, a murine lymphoblast grown in vitro seemed to contain a factor similar to that found in thymic extracts. The factor was not found when the L-5178Y was grown in vivo. Subsequently, the normal ratios of lymphocytes to polymorphoranuclear leukocytes changed, and the mice became refractory. It was concluded that the test method was subject to too many fluctuations and could yield conflicting data. In an addendum, Metcalf points out some of the external factors that influence L/P ratios in young mice.