The rate at which three different species of a fluid phase marker (horseradish peroxidase) reached the lysosomal compartment of mouse LM fibroblasts was compared. Initial observations suggested that the rate was a function of particle size. Adjustment of the concentrations of the various species of HRP to equal numbers of physical particles negated the apparent affect. Thus, particle size does not affect the intracellular routing of a ligand entering fibroblasts by non-adsorptive pinocytosis.