The paper presents a method for the estimation of the bias errors of active and passive sensors used in connection with multisensor multitarget tracking. It is based on comparing the measurements of targets of opportunity, and does not require reference targets or reference sensors. The method handles bias errors that vary with time, and is suitable for on-line processing. The most essential ingredients of the method are: Including of a priori values and uncertainties; minimization of the appropriate function; linearization around nominal points; introduction of process noise; and quasi-recursive processing. Bias estimation is often difficult because of the limited observability of sensor biases, in the sense that there amy not be a unique set of biases that explains the relative errors between measurements. This problem is discussed, and it is illustrated by simulations how the presented method avoids these problems.