In a paper recently published in this journal, Philpot and Wells (1) reported that they were unable to confirm some earlier experiments by the present writers which described the isolation and some biologic properties of a lipid extract from young cultures of virulent tubercle bacilli. The discrepancy be tween the results of Philpot and Wells and those of the present writers made it seem advisable at this stage to make a report on the progress of the work in this laboratory.

The lipid material in question had been termed “cord factor” because it was obtained from “cord-forming” strains of Mycobacteria (virulent tubercle bacilli and BCG). As first described (2), it was a crude petroleum ether extract from young, living, surface-grown cultures which had two characteristic biologic properties. It inhibited the migration of leukocytes in vitro and exerted a de layed toxicity for mice which would become manifest after repeated intra-peritoneal injections of relatively small doses. The latter activity seemed to lend itself to assaying the material biologically, especially for quantitative deter minations during the process of chemical purification, because no other constit uents of the tubercle bacillus are known to have a similar toxic action. This is in contrast to the inhibitory action on the migration of leukocytes which can be exerted by various bacterial products (3, 3o-3c). As a consequence, the latter technique is of questionable value in tracing the toxic material during chemical fractionation.