The mean death time in a cloacal test (MDT/CT) was employed to define the pathogenic type of isolates of the Newcastle Disease virus (NDV) with 5-week-old birds. The results were compared with the use of standard methods for typing--the mean death time with 10-day-old chick embryos as defined by the minimal lethal dose (MDT/MLD), the intracerebral pathogenic index for day-old chicks (ICPI), the intravenous pathogenic index for 6-8-weak-old birds (IVPI). Sixteen local isolates and 6 reference strains of NDV were used in the experiments. Using the author's modified technique to determine MDT/CT the velogenic strains of NDV were strictly differentiated from the local meso- and lentogenic NDV isolates and reference strains. At the same time differentiation was made of the velogenic-viscerotropic and of the velogenic-neurotropic isolates (strains). The values of MDT/CT in the experiments with isolates of the velogenic pathotype (14 in number) varied from 4.0 to 7.8 (mean = 5.32 +/- 0.26), and by them the isolates were graded for virulence within the respective (viscero- and neurotropic) group. For the lentogenic (5) and the mesogenic (3) isolates and strains these values were approx. 10. With 6 local NDV isolates of the velogenic-viscerotropic pathotype the test birds showed edema of the head and neck, with swelling of the eyelids, and post mortem the accumulation of semi-liquid fibrinous mass under the skin in these parts of the body and conjunctivitis were found. MDT/CT in the modification employed proved simple and readily applicable, and superior to IVPI in reliability, and is, therefore, suggested for use in typing and differentiating NDV isolates.