A system is described for quantifying the spatial distribution of inhaled particles in complete lobar histological sections of rodent lung. It is based on a semiautomatic image analyser, linked to a readily available microcomputer. Application of the system to quantify the aggregation of particles, and their distances from airways and lung periphery is described. The observed distributions are compared with the expected distributions from a random set of particles, and the significance of the difference is calculated. Each statistical test is illustrated by an example.