The field - field correlation function and the conditional covariance for the ionic microfield in a plasma are calculated within a virial cluster expansion of the two-time generating function. Results for the Debye potential are given. The short-time, high-field behaviour is discussed within a regularization procedure. The evaluated conditional covariance provides direct access to the probability density of a field strength jump which appears in the well known model microfield method. The resulting stochastic process would describe the time evolution of the ionic microfield on the basis of the real motion of the plasma particles instead of assuming Markovian behaviour.