The response of macrophages from the livers and spleens of mice given a single immunizing dose of T4 bacteriophage has been studied. Following their rapid removal from the circulation, phage particles were found to be concentrated in the liver to a level twelve times that for the spleen. Investigation of the fate of ingested phage showed that it was disposed of more rapidly in the liver than in the spleen, as measured by the disappearance of viable T4 particles and by the loss of radioactive label following injection of [131I]T4. It was also found that antigen-containing Kupffer cells could elicit little or no antibody synthesis on transfer into normal syngeneic recipients, or on incubation with lymphoid cells in vitro. It is suggested that these macrophages differ from other components of the reticulo-endothelial system in their treatment of T4 antigen, and may be concerned mainly with its breakdown and disposal rather than with providing a stimulus for the initiation of antibody synthesis.