Leukopenia (WBC 1.9 x 10(9)/L) was reported from an automated cell counter (TOA E-5,000, Symex) on a patient blood sample collected in ethylene diamine tetracetic acid (EDTA). Leukocyte aggregates of 30-50 cells were seen on the peripheral blood smear. Blood samples collected in heparin, citrate, and EDTA had white blood counts (WBC) of 8.1, 8.1, and 2.5 x 10(9)/L. A 0.3-mL aliquot of patient EDTA-plasma mixed with 0.3 mL normal packed cells (NPC) resulted in a WBC of 2.7 x 10(9)/L, a reduction of 58% from the control value. Increasing concentrations of EDTA (0.35-2.16 mg/mL), patient serum, and NPC resulted in a greater than 60% WBC reduction at EDTA concentrations greater than 0.96 mg/mL. Serial dilutions of patient serum in phosphate buffered saline (PBS) showed greater than 50% WBC reduction at a dilution of 1:4. Incubation (0, 1, and 2 hours) (0 degrees C, room temperature [RT], and 37 degrees C) of samples containing patient serum, EDTA, and NPC resulted in the greatest WBC reduction at 2 hours and at RT (60%). Incubation with 0.01 M dithiothreitol (DTT) abolished the ability of patient serum to decrease the WBC. These data suggest an EDTA-dependent, low titer IgM leukoagglutinin.