The nucleic acid and protein contents of the various subcellular fractions of rat spleen homogenates were determined at varying intervals of time following a single intravenous injection of sheep red blood cell membranes. Small differences were recorded in the NA content of the different fractions, as compared with controls. The microsomal fraction was the only one to exhibit significant increases in NA content. The protein content of the various subcellular fractions did not show significant deviations from control values. Antibodies could be eluted from microsomes of spleens of immunized rats by sodium deoxycholate and ribonuclease treatment. A quantitative difference was found in the immune response of the two strains of rats used (R.V.H. and Wistar rats).