“…Antigenic variations among the pleuropneumonia-like group of microorganisms (PPLO), defined herein as including various "L" forms of bacteria and the Mycoplasmataceae, have been repeatedly demonstrated by various serological reactions (Bailey et al, 1961;Card, 1959;Clark et al, 1963;Coriell, Fabrizio, and Wilson, 1960;Klieneberger, 1938Klieneberger, , 1940Malizia, Barile, and Riggs, 1961). This, together with rather striking host specificity and often tissue localization, suggests that some physicochemically demonstrable differences may exist in the proteins of the different species.…”