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In the present investigation, determinations of the duration of mechanical systole and left ventricular ejection were made from simultaneous recordings of the heart sounds and the carotid or axillary arterial pulsation in older children and in mature and premature neonatal infants without cardiovascular disease. In older children and mature neonates with heart rates ranging from 69 to 196 beats/min., both duration of total systole and left ventricular ejection time diminished linearly and in parallel with increasing heart rate up to a level of 150 beats/min. At high levels of heart rate the phases of systole diminished more gradually with increasing heart rate. The isovolumetric contraction time tended to remain constant throughout the range of heart rates studied. Among the premature neonates there was a wider distribution in the measured intervals, and left ventricular ejection time tended to be higher at any level of heart rate. The prolonged ejection time in this group can in part be attributed to the presence of anemia.

SUMMARY Procedures are described for routine isolation and production of nonproducing Rous sarcoma cells (NP cells) and for use of these cells in the NP activation test for assay of avian leukosis/sarcoma viruses. The NP activation test was found to be sensitive, reproducible, easy to perform, and highly reliable. It required only 5-7 days of cultivation for the activation phase, and 8 days for the RSV assay phase. The sensitivity of the NP activation test for detection of avian leukosis viruses appeared comparable to the COFAL and RIF tests. Because of its relative simplicity and shorter culture periods, a far greater number of samples can be tested in the same period.