Chemotaxis of polymorphonuclear leukocytes (PMN) in 100%, 50% and 25% whole blood as well as in separated preparations were compared using both N-formyl peptide and interleukin-8 as chemoattractants and Boyden-type chambers with 'sparse-pore' polycarbonate (Nucleopore, California) membranes. In addition, attachment, spreading and 'drop-off' of cells from the 'sparse-pore' membrane were documented. In whole blood, fewer PMN attached to the membrane, but these showed greater chemotactic activity and exhibited more spreading and more 'drop-off' than separated PMN. Despite these differences, the 'sparse-pore' polycarbonate membrane system may be used to assay chemotaxis with whole blood as the source of PMN.