The murine macrophage-derived cell line P388D1 is commonly used for the production of Interleukin 1 (IL1) and other macrophage products. In order to circumvent the tendency of this cell line to undergo selection for a non-adherent subpopulation of cells which does not produce IL1, we have grown P388D1 cells attached to microcarrier beads. IL1 produced by these cells is indistinguishable from that elaborated by cells grown in monolayer or suspension culture. Moreover, several volumes of conditioned medium can be produced from one batch of cells suitable for the large-scale purification of this mediator. © 1981.