Summary: A method for the maintenance of mouse spleen cells in vitro by means of a rabbit kidney cell feeder layer has been described. Measuring cell viability by trypan blue exclusion, survival of spleen cells after 24 hr in culture was improved from approximately 50% to greater than 90%. Functional integrity of spleen cells cultured during that time interval was unimpaired as measured by the production of interferon and synthesis of antibody on transfer to x-irradiated recipient mice. However, the ability to form antibody plaques on agar was reduced, though not abolished, by incubation of spleen cells in vitro.