An evaluation of a Coulter Counter Model S880 has been carried out. Certain findings have been further evaluated on a second instrument. Several safety hazards have been identified which could be corrected by minor modifications of the instrument or the mode of operation. Precision was found to be generally satisfactory, though for the platelet count and the WBC it was sometimes worse than specified. Most parameters were not strictly linear on dilution but only for the MCV and the MCH was the deviation from linearity sufficient to be of practical importance. Carryover was satisfactory. Results were generally comparable with those of a Coulter Counter Model S Plus IV. Estimates on all parameters were satisfactory on blood samples stored up to 24 h at either 4 degrees C or room temperature; beyond 24 h inaccuracy was seen particularly in the WBC, platelet count and MCV. Throughput was 60 to 70 samples/h in the whole-blood mode. Start-up and shut-down times were acceptably short. The instrument was assessed as acceptable and clinically useful, although reliability was somewhat less than expectations.