SummaryThe binding of lipid to proteins isolated from rat liver was investigated by incubating ultrasonically dispersed mixtures of phospliolipids and neutral lipids with3H‐lahelled proteins. After incubaation the lipid‐protein eomplexes were separated from the imbound protein by preparative ultracen‐ trifugation at d = 1·21.Gonsiderable binding of soluble (105,000 × g supernatant) and “solubilised” membrane (endoplasmic reticulum) protein to lipid occurred after incubation for 90 min. at 37°, whereas plasma proteins under similar conditions were not significantly bound to lipid. These observations suggest that the binding of liver protein to lipid, as measured in these experiments, was due to specific binding of liver proteins with lipid, rather than the result of non‐specific adsorbance. Alteration of the phospholipid: neutral lipid ratios appreciably influeneed the amonnt of protein bound, and the lipid‐protein interaction was found to be dependent on time and temperature.