In recent years it has become obvious that the problem of bacterial variation is vastly more complex than was at first believed. It appears that bacteria possess several methods of variation or ability to alter their morphologic and biologic nature. One mode of variation concerns the widely studied culture phases commonly regarded as the mucoid (M), smooth (S) and rough (R) phases of a given type, one involves the apparent change from coccus to bacillary form or vice versa, another the development of translucent and lytic forms, and still another concerns actual type transformation. All four modes of variation were observed in studies on a strain of Micrococcus tetragenus (Reimann, 1936, a, b, c, d.) The possibility of typo transformation has been suggested for years, especially in regard to the meningococcus and gonococcus (Atkin, 1925) and among the colon-typhoid group of bacilli. The question has been of especial interest in regard to the relationship of small-pox virus to vaccinia virus, and between the so-called "street" and "fixed" virus of rabies. Changes involving the conversion of pneumococci into streptococci have not been generally recognized although the recent studies of Paul (1934) show that certain R dissociants of pneumococci are indistinguishable from certain strains of Streptococcus viridans. It 'LThis work was done in the Department of Medicine, at the University of Minnesota and was aided by a grant from the Graduate School Fund.