Summary The experiments reported give further support of the possibility of transferring morphine tolerance by administration of material extracted from the brain of tolerant animals. However, for reasons still unknown, brain extracts identically prepared from donors subjected to the same treatment show widely varying potencies. To demonstrate the effect of low potency extracts, higher doses have to be given with a longer interval between administration of the extracts and testing of their effect. Not all tests are equally suitable for showing the transfer of tolerance: graded responses are less satisfactory than quantal effects. The shift in the mortality curve of morphine, although less sensitive than the analgesic effect, is particularly suitable for an easy demonstration of the transfer of tolerance. Purification of the transfer factor, although still incomplete, has reached the point where administration of 0.1 to 0.2 μg of material per mouse induces significant tolerance to the analgesic effect of morphine.