Radiation response of the serial isolated collector vertical pnp transistor in the voltage regulator L4940V5 is a complex function of the radiation-induced trapped charge concentration, load current, bias voltage and negative feedback reaction. Input bias voltage and oxide trapped charge above the base area improved radiation hardness of the serial transistor, whilst the load current and interface traps above the base decreased radiation tolerance of the examined devices. Additive radiation effects of interface traps and oxide trapped charge above the emitter area had a negative influence on radiation hardness of the serial pnp transistor. Negative feedback reaction heavily affected operation of voltage regulator L4940V5, causing a significant shift of the serial transistor's operating point on the characteristic beta(V-BE). An analogy was perceived between characteristics of the serial transistor's base current in a moderately loaded voltage regulator L4940V5 and McLean's curves on variation of the interface traps concentration in a biased oxide.