The likely origin of the New Zealand grapefruit (NZGF) is discussed and present and future trends in its production and utilisation presented. Early and late season samples of NZGF juice were analysed for the presence of the enzymes pectinesterase, polygalacturonase and ascorbic acid oxidase, no trace of the latter two being found. Samples of juice from NZGF harvested at regular intervals from July until December 1973 were analysed for yield, total soluble solids, titratable acidity, pH, pectinesterase activity, and ascorbic acid content. The average yield of juice obtained (35.6% w/w) was significantly lower than that reported from overseas for true grapefruit. The level of total soluble solids remained fairly constant in the range 12.0 to 12.6%, while the pH of the juice increased throughout the season from 2.95 to 3.40. The titratable acidity was within the range 1.0 to 2.0 grams of citric acid per 100 ml of juice, while the Brix : acid ratio varied from 5.02 to 10.03. The level of pectinesterase in the juice (which increased as the season progressed) was comparable with that found in overseas citrus juices, while the level of ascorbic acid in the juice declined over the season from 32.4 to 23.2 mg/100 ml, in agreement with overseas trends. With the exception of yield, the compositional characteristics of NZGP juice reported here do not differ markedly from overseas grapefruit juices. The important role which pectinesterase plays in the destabilisation of citrus juice cloud is outlined and possible methods for inactivating the enzyme are described. As the application of heat is the only method in commercial use, factors affecting and methods for studying the thermal inactivation of enzymes are discussed. As the major objection to most of these methods is the way in which the heating and cooling lags are evaluated, a new method which adequately describes these thermal lags has been developed for determining the thermal resistance of pectinesterase in NZGP juice. A digital computer was programmed to determine (using a trial and error technique) the constants in two expressions which relate the equivalent effect of unsteady state heating and cooling of NZGF juice to the inactivation of pectinesterase. One expression assumed that the rate of inactivation was exponentially related to temperature; in this case the constant was the z value. The other expression assumed that the rate was related to temperature according to the Arrhenius equation, in which case the constant was the activation energy. The two constants were evaluated for both low and high pH juice. It was found that the latter expression using the Arrhenius equation described the change in rate of inactivation with temperature more adequately than the former expression. From these expressions the times required at different temperatures to inactivate pectinesterase in NZGP juice of varying pH were calculated. The application of these results to the industrial processing of NZGP juice is discussed.