The effect of various chemotactic factors on phagocytosis of sheep erythrocytes sensitized with IgG and complement (EAC1423) by human neutrophils (PMN) was studied. The bacterial chemotactic factor, a butanol extract of an Escherichia coli culture filtrate, the pronase sensitive and insensitive chemotactic substances isolated from this extract, C5a, a chemotactic fragment of the fifth component of complement, and a chemotactic synthetic peptide, formlymethionyl-leucine all significantly reduced the initial rate of ingestion at concentrations that are chemotactic for PMN. The bacterial chemotactic factor produced a 45 to 75% decrease in the initial rate of ingestion, but did not affect adherence of the EAC1423 and PMN. This same substance had no consistent effect on the total number of EAC1423 ingested at 90 min or longer or on the time interval from the addition of the EAC1423 to the start of ingestion. The inhibitory effect of the bacterial factor increased as the amount of bacterial factor was increased and the inhibitory effect was reversible. Inhibition by bacterial factor could be decreased by increasing the concentration of EAC1423; and if the concentration was increased 3-fold, the inhibition was abolished. These results suggest that inhibition by bacterial factor is competitive in nature.