Data for bulked samples of ungraded Cox's Orange Pippin fruit obtained from many sources over a period of 18 years show that there is a general inverse relationship between mean mass per apple and mean calcium concentration. Part of the scatter around the regression curve was accounted for by differences between years but there were larger differences between samples of the same size picked from different orchards in the same year. Even with the most favourable conditions the observed samples with mean mass per apple greater than 150 g had less than 5 mg Ca/100 g when no calcium sprays were applied and in general most samples with mean masses per apple greater than 110 g were likely to have concentrations of calcium below this and hence were liable to develop bitter pit. Fruit calcium level was a more important determinant of liability to bitter pit than was mean mass per apple.Although there was some evidence of the same relationship between calcium concentration and mass when apples from the same tree were analysed individually other factors often negated it. Two-to three-fold differences in calcium concentration were observed in individual apples from the same tree.