When particles of Type B are a subclass of particles of Type A, the number of B-particles in a liquid may be estimated by taking the product of (i) the count of A-particles in a sample and (ii) the fraction of B-particles determined by differentiation of a fixed number of A-particles. Confidence intervals for the number of B-particles are derived. The effect on the estimator of varying the volume and/or the number of differentiated A-particles is investigated.