Marked differences were noted among 14 antibiotics in their filterability through collodion (nitrated cellulose) membranes. The findings could not be attributed to differences in molecular weight or degree of serum protein binding, although the latter factor imposed an additional restraint on the passage of certain agents. Poorly filtered antibiotics appeared to be absorbed by the collodion. Some of these observations might be accounted for by the known ability of nitrated cellulose to interact with anions. However, the extreme differences in filterability among the various penicillins and the very restricted passage of certain cationic antibiotics appeared to be better explained by intrinsic differences in the diffusibility of these agents through water. Although differential filtration offers a means of assaying samples containing certain mixtures of antibiotics, the degree of separation exhibited by collodion membranes is not sufficient to make them useful for clinical purposes.