Time-lapse films of particle motion on the leading lamella of chick heart fibroblasts and mouse peritoneal macrophages were analyzed . The particles were composed of powdered glass or powdered aminated polystyrene and were 0.5-1 .0 /um in radius . Particle motions were described by steps in position from one frame of the time-lapse movies to the next . The statistics of the step-size distribution of the particles were consistent with a particle in Brownian motion subject to a constant force . From the Brownian movement, we have calculated the two-dimensional diffusion coefficient of different particles. These vary by more than an order of magnitude (10-11 -10-1°cm'/s) even for particles composed of the same material and located very close to each other on the surface of the cell . This variation was not correlated with particle size but is interpretable as a result of different numbers of adhesive bonds holding the particles to the cells. The constant component of particle movement can be interpreted as a result of a constant force acting on each particle (0 .1-1 .0 X 10 -e dyn) . Variations in the fractional coefficient for particles close to each other on the cell surface do not yield corresponding differences in velocity, suggesting that the frictional coefficient and the driving force vary together . This is consistent with the hypothesis that the particles are carried by flow of the membrane as a whole or by flow of some submembrane material . The utility of our methods for monitoring cell motile behavior in biologically interesting situations, such as a chemotactic gradient, is discussed .It has been shown by a number of authors (1,12,15) that, when the leading edge of a moving cell contacts a small particle, the particle is frequently "picked up" by the cell and transported backward over or under the leading lamella. If transported on the upper surface of the lamella, the particles usually come to rest at the margin of the lamelloplasm and the granular endoplasm. Many different cell types (e.g ., fibroblasts (5), epithelial cells [9], and nerve cells [4]) have been shown to transport particles in this manner . Furthermore, particles composed of many different materials (e.g., polystrene, glass, charcoal, gold, and cancanavalin A-treated erythrocytes) can be transported.Detailed photographic records of the tracks of particles as they are transported over the leading lamella have been reported by several authors (1, 11,12) . In addition to the general transport of the particles from the front toward the back of the cells, these records reveal that the individual positional changes undergone by the particles during fixed time intervals are highly variable . This result indicates the existence of both constant and random components to the forces causing particle motion.When the leading lamella contacts a particle, it has no way to tell whether or not the particle is fixed to the surface over which the cell is moving. Consequently, the forces that pull particles backward over the leading lamella are p...