The author emphasizes the most important characteristics of the cascode from the point of view of its usefulness as a biological amplifier: high gain, bandwidth, low input capacitance, small number of components and facility in supplying. A d.c. amplifier with symmetrical cascode as input stage is described. A gain of 5-5000 is obtained with only the symmetrical cascode and a single ended compressor stage. A differential potentiometer controls the static level of cascode by balancing the additional current that increases the mutual conductance in the lower triodes. Dynamic balance is by grid input biasing afforded by a potentiometer between the input cathodes. The cascode is long tailed by a pentode, the feedback for in-phase signals increasing rejection. The discrimination factor in practice amounts to 2×104.