Lankford, C. E. (The University of Texas, Austin), James R. Walker, James B. Reeves, N. H. Nabbut, B. R. Byers, and R. J. Jones. Inoculum-dependent division lag of Bacillus cultures and its relation to an endogenous factor(s) ("schizokinen"). J. Bacteriol. 91:1070-1079. 1966.-When cells of Bacillus megaterium, grown on Brain Heart Infusion Agar, were inoculated into a chemically defined medium, they exhibited a division lag which was an inverse function of inoculum size. The addition of filtrates of cultures from the same medium eliminated the inoculum-dependent component of lag, but not an inoculum-independent residual lag of constant duration. Culture filtrate of B. subtilis var. niger not only eliminated its inoculum-dependent lag but also was required to sustain exponential division. Dose-response and "growth time" bioassays, developed to measure lag-reducing activity of filtrates, demonstrated accumulation of active filtrate factor to a "critical" concentration prior to division initiation. Addition of this concentration to cultures eliminated the inoculum-dependent lag. Accumulation of the factor ceased temporarily at onset of division, but excretion was resumed later during exponential growth. Accumulation of a lag-reducing, cell-associated factor followed a similar course. Chromatographic and bioautographic analyses of culture filtrates of B. megaterium indicated that a single substance was primarily responsible for their activity. Results of dose-response tests for reciprocal activities of filtrates of different Bacillus species and strains suggested production of different factors by some, and of different quantities of similar factors by others. It is proposed that such endogenous factors which are synthesized and accumulate to a population-dependent concentration as a requisite to initiation and maintenance of division be designated as "schizokinens."