By investigating the growing use of plastics within Mongolian dairying, this paper explores emerging microbial/social assemblages as they relate to local and imported ideas of purity and hygiene. Although many Mongolian herders prefer to use dairy equipment made from materials such as wood and hide, these items are increasingly being replaced by plastic ones. As new infrastructure connects northern herders to more extensive markets, it presents challenges for herders and for the microbial communities with whom they co-exist, placing herders under increasing pressures to compete with large-scale dairy enterprises that brand, package and distribute standardised dairy products. Looking at the changing material culture of Mongolian dairying and its relationships with microbial communities, this paper examines two emergent notions of purity: the first in which sterility is generated and contained and the second in which living dairy is harnessed and grown.