Sera from cholestatic patients have been found to interfere with lymphocyte rosette formation (E rosettes). Lymphocytes from patients with cholestasis showed a lower percentage (42.36%) of cells binding sheep red blood cells in comparison with a control group (62%). In the presence of cholestatic serum the number of E rosettes formed by lymphocytes from normal subjects declined to 44.16%. This behaviour, however, can reverse, since lymphocytes from cholestatic patients previously incubated with normal serum, showed a nearly normal capability of forming E rosettes (57.90%). Therefore, it is conceivable that among the several factors which are present in sera from patients with either chronic or acute hepatitis and which are able to interfere with T-lymphocyte function, there may be included also the factor which is present in sera from cholestatic patients. The possible role of bile salts and hyperlipoproteinaemia present in those patients requires further investigation.