THE most difficult aspect of determining the specific “trace element” requirements of micro-organisms is the preparation of a medium free of these factors. With some micro-organisms, EDTA chelates form a reserve from which cations may be drawn1, but we have found that L. acidophilus cannot utilize EDTA chelates. When the trace elements are tied up by EDTA, a medium in which L. acidophilus normally grows well no longer supports growth. Moreover, the EDTA portion of the molecule is not toxic per se, because the addition of excess cations provides the usual growth.