1. The first lactation shews greater variability than the second, third, fourth or fifth. 2. The estimation of one lactation from another cannot be made with great accuracy, since the correlation coefficient between even successive lactations does not rise above + ·6. 3. It is necessary, in classing a cow, to decide on one lactation that shall represent her mature capability. It is suggested that the maximum lactation is the most suitable one to chose. 4. The correlation coefficient with the maximum lactation increases from ·394 for first to ·762 for fifth lactation. 5. The mean of the first and second lactations however gives a correlation coefficient of ·526 with maximum lactation, which is higher than any of the first three taken separately.