The clearance capacities of reversed phase silica gel, widely used in analytical chemistry, were studied in vitro. The plasma clearances of amitriptyline, quinidine, digoxin, digitoxin, methaqualone, phenobarbital and phenytoin were determined at typical toxic plasma levels. Between 88.8 and 99.5% of the drugs were eliminated from plasma by reversed phase silica gel, which compares favorably with amberlite XAD-4 and coated activated charcoal. The biocompatibility of reversed phase silica gel was also evaluated in vitro. Thrombocytopenia and leukopenia were noted while coagulation parameters, electrolytes, acid-base balance, glucose, urea, creatinine, uric acid, total protein, albumin, bilirubin, cholesterol and triglycerides were not significantly affected. Preliminary results in vitro support the high clearance capacity of reversed phase silica gel and suggest a biocompatibility similar to that of other sorbents currently in use. Further investigation of the clearance capacities and biocompatibility of reversed phase silica gel seems justified.