SUMMARY The cytotoxic effect of detergent-laden Millipore filters to cells cultured in diffusion chambers is demonstrated. Lymph node lymphocytes are sensitive to the toxicity while the tumors tested are unaffected. Washing the filters by boiling in distilled water prior to use significantly increased culturing efficiency. Washing plus incubation in 10% syngeneic mouse serum does not increase culturing efficiency over washing alone.