The ratio of mouse to rat albumin secreted by mouse hepatoma x rat hepatoma hybrid cells is constant (of the order of 5.0) irrespective of the total amounts produced. The present results establish for seven independent hybrid clones that the coordination in the ratio of mouse to rat product applies also at the level of accumulation of albumin mRNAs of the two species. The interpretation that coordinate synthesis reflects coordinate transcription of the relevant genes is thus reinforced.