A new variant subline was selected from S49 mouse lymphoma cells grown in suspension culture. These cells, designated S49-Adh, demonstrate increased adhesiveness to their substratum. This, in turn, allows them to grow as a monolayer. Once a dense monolayer is formed, daughter cells develop as a suspension borne population while still retaining their substrate adhesiveness. Cell substrate adhesiveness is a stable genetic trait insofar as submitting S49-Adh cells to both extended periods of in vitro cultivation in culture medium, or in vivo selection through passage in nude mice didn't change their substrate-adhesive phenotype.