Staphylococcus epidermidis infection of plastic catheters is often associated with heavy deposits of slime. To test whether this slime affects the human cellular immune response, its effect on the lymphoproliferative response of mononuclear cells to polyclonal stimulators was measured. Slime drastically reduces this response. Its inhibitory action was not immediate but took place over a few days and resulted in destruction of affected cells. The effect is dose related. This inhibition of cellular response may contribute to S epidermidis infection of implanted prostheses.