A composition comprising leukotoxin proteins isolated from a bacterium is provided. In this composition, greater than 85% of the leukotoxin proteins are chemically modified at a basic amino acid residue, and the proteins induce cell death in myeloid leukocytes, while remaining substantially non-toxic to lymphoid leukocytes, lymphocytes, and red blood cells. Also provided is a method of selectively inducing cell death in myeloid leukocytes. The method comprises contacting the myeloid leukocytes with a composition comprising leukotoxin proteins. These leukotoxin proteins may be isolated from the NJ4500 strain of Actinobacillus actinomycetemcomitans. A method of purifying leukotoxin protein from the NJ4500 strain of Actinobacillus actinomycetemcomitans is also provided, as well as an assay that allows for the rapid determination of the activity of a given drug against leukemic cells either taken from a patient or derived from a cell line. The assay is performed in the presence of whole blood or serum.