Although the modifying effect of environmental factors on bacterial variation (dissociation) has been widely studied (Braun, 1947), there has been a paucity of data concerning the influence of the trace metals on this phenomenon. In the course of an investigation of factors that influence variation in Brucella abortu8, our attention was directed to the possible role of the trace metal ions by the finding that during prolonged growth of smooth cultures of B. abortus in broth no nonsmooth variants (e.g., R, M) could be observed when 0.03 M Na-pyrophosphate was added to the medium. In contrast, similar cultures without pyrophosphate showed a progressive establishment of nonsmooth variants (mutants). Since pyrophosphate forms complexes with many metal ions and is known to inhibit certain enzymes (Sumner and Somers, 1947), the present study was undertaken in an attempt to elucidate the mechanism of action of pyrophosphate as well as of trace enetals in bacterial variation. METHODS A description of the procedures employed in the study of variation (dissociation) in B. abortus has been published previously (Braun, 1946). The method is based on the determination of the dissociation index (D.I.)2 of a clone (2583) of B. abortus (originally isolated from strain 19, U. S. Bureau of Animal Industry), cultured in buffered beef extract broth (pH 6.8) under standardized