A search for high molecular residues in penicillin G was carried out using several recently described methods. By the membrane filtration method the protein content of a large number of batches of penicillin G, 6-APA (chemically prepared), and ampicillin was determined. Significantly lower protein values were found for these compounds than for microbiologically produced 6-APA and the ampicillin made from it. Purified macromolecular complexes of penicillin gave protein values in the membrane filter method. Our results indicate that the protein content of most commercially available benzylpenicillins is negligible.