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In a single-mutant strain, mutations may occur in other genes for resistance; and when two mutant genes are together in one individual (double mutants) their effect is cumulative. Moreover, it happens that the resistance of a double mutant is higher than the sum of resistances of two single mutants. If a third gene for resistance, a fourth, etc., mutate in the same line, the combined effect of all these mutations is a high degree of resistance or complete resistance.

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Eleven healthy, adult, male volunteers were exposed on separate occasions to average levels of sulfur dioxide of 1, 5, and 13 ppm. The subjects were seated in a body plethysmograph, breathing spontaneously by mouth while measurements of respiratory mechanics were made with an esophageal catheter. Exposures lasted 10–30 min and, for each subject, were spaced at least 1 month apart. With one exception the group showed no significant increase in pulmonary flow resistance at the lowest concentration of gas. At both 5 and 13 ppm, flow resistance was elevated, the change being greater at 13 ppm. The change occurred within 1 min of exposure, increased after 5 min but, on the average, showed no further change after 10 min. Four subjects were exposed to 5 or 13 ppm of SO2 for 30 min without exhibiting increases in flow resistance beyond the 1st 10 min. There were no consistent changes in pulmonary compliance, tidal volume, breathing frequency, or pulse rate. The functional residual capacity increased slightly during exposure to 13 ppm. Note: (With the Technical Assistance of Eleanor Root) Submitted on May 24, 1961

The method for the estimation of the relative nutritive value (RNV) of protein utilizing the slope-ratio assay with young rats has been described in detail. A computer program has been written for the easy evaluation of the validity and precision of the results obtained. Comparisons of the results obtained with weight gain, body water, and body nitrogen as measures of response demonstrate generally similar results. In view of the ease with which body water may be determined, it is probably the metameter of choice. The relative precision which may be expected when six proteins are assayed utilizing different numbers of animals per test protein has been determined. As few as nine animals per test protein (three animals per group, three levels of each protein) yield reasonably satisfactory results.

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