When the following compounds are added to cultures of chicken heart fibroblasts, vacuoliza-tion of the cell plasma is produced: NH3, NH4Cl, methyl-amine HC1, n-propylamine HC1, iso-propylamine, n-butyl-amine HC1, benzylamine, [beta]-phenylethylamine, piperazine and piperidine. Under the same conditions, no vacuolization is produced by: Aniline, pyridine, quinoline, an-thranilic acid methyl ester, o-aminobenzoic acid, m-amino-benzoic acid, tyrosine, dioxyphenylalanine, arginine, choline chloride, acetyl choline, aneurine, tetramethyl NH4I and tetraethyl NH4I. Cultures kept at room temp. may show vacuolization because the NH3 present may not be completely used for synthetic processes and an excessive amt. accumulates in the medium.