AbstractThe time to maturation of a free‐living, protandrous, self‐fertilizing nematode,Caenorhabditis briggsae, was used to assay preparations of a proteinaceous growth factor and the effects of different treatments. Development was measured as time of the production of F1offspring against concentration of growth factor. The data were transformed to the reciprocal of F1time against logarithm of the dose and analyzed by regression analysis. Estimates were made of the dose of growth factor required for maturation in seven days, and relative potencies were determined for assays whose regression coefficients did not significantly differ from one another. The dose of growth factor required for maturation in seven days fell within the range from 8 μg/ml to 28 μg/ml. The methods of treatment for activation of growth factor studied were freezing overnight at —23°C, dialysis to low phosphate, and addition of the sucrose polymer, FICOLL. Reproducibility of results was shown for controls made for each experiment and for assays repeated at different times. Inocula of various numbers of larvae per tube, 1 through 30, were compared; significant increase in maturation time occurred when inocula were large