Sonicated dispersions of phosphatidylcholine, lysophosphatidylcholine, phosphatidylinositol, and phosphatidylserine in deuterium oxide were studied by NMR, and the changes in spectra produced on addition of penicillin G and ampicillin to the dispersions were observed. The antibiotics appeared to reduce the configurational freedom and kinetic motion of the phospholipid hydrocarbon chains due to hydrophobic interaction between the hydrocarbon chains and the phcnyl portion of the antibiotic side chain. Penicillin G interacted with the phospholipid systems to a greater degree than ampicillin, while the phospholipids interacted in the order phosphatidylserine > phosphatidylcholine > phosphatidylinositol > lysophosphatidylcholine. Keyphrases 0 Antibiotic interactions with phospholipids--studied using NMR spectroscopy 0 Phospholipids, interactions with penicillin G and ampicillin--studied using NMR spectroscopy 0 Penicillin G and ampicillin interactions with phospholipids--studied using NMR spectroscopy 0 Ampicillin and penicillin G interactions with phospholipids---studied using NMR spectroscopy 0 NMR spectroscopy-used to study antibiotic interactions with phospholipids This work is part of a physical study ( I ) designed to investigate the reported differences in in uiuo activity (2) of penicillin G (I) and ampicillin (11) by attempting to correlate these differences with the relative penicillin affinity for binding with phospholipids ( I l k , 1116, IIIc, and IIld). Previous work examined the effects of interactions on the rheology of phospholipid systems (3, 4), on diffusion of the antibiotics through phospholipid dispersions, and on gel filtration (5). The significance NH,)-COOH, R , a n d R, = f a t t y a c i d c h a i n s (Cl1-Cl8) Potassium penicillin G was the gift of Glaxo Research Limited.