An investigation was carried out on 30 diabetic patients in an attempt to clarify the relationship between serum biguanide levels and raised lactate. No consistent relationship was demonstrable between the serum biguanide level, administered dosage and time of administration. There was also no correlation between biguanide and lactate increase. It is not justifiable to quote a specific serum level of biguanides in defining lactic acidosis. A causal association between biguanide medication and lactic acidosis seems to be possible only by determination of serum and tissue levels. Determination of biguanide levels was carried out in the serum and tissue of a patient who had died as a result of lactic acidosis after phenformin administration. While the serum levels were only slightly higher than the therapeutic range, both liver and kidney tissue showed highly toxic levels. Furthermore, the amount of biguanides in the body was calculated in another patient successfully treated for lactic acidosis after buformin therapy. A differentiation should be made between biguanide-induced and biguanide-associated lactic acidosis. In both forms serum levels can be within relatively low ranges. In the former condition, the biguanides alone are responsible for the development of lactic acidosis by blocking the respiratory chain. In the latter condition they aggravate an already existing pathological condition, and can, therefore, represent a lethal factor.