The activity index (k) is defined as the amount of cement that can be replaced by 1 part of fly ash (or a similar material), without decreasing the strength of the mortar (or concrete). When determining k for fly ash, when used in mortar, it is necessary to know the relation between the water-cement ratio and the compressive strength. Such relations has been determined for mortar at 3, 28 and 91 days and for three different Swedish cements (ordinary Portland). In order to vary the water-cement ratio without affecting the consistency of the mortar, different amounts of the cement were replaced by corresponding amounts of inert powder of quarts. The relation between the water-cement ratio and the compressive strength seems to be well described by the relation according to Bolomey, f = A (C/W-B), where W/C is the water-cement ratio and A and B are constants. B seems to decrease from about 0,53 to about 0,47 when the age of the mortar increases from 3 to 91 days and A seem to be, for the cements used in the investigation, in the intervals 20-25 MPa at 3 days, 30-35 at 28 days and 35-40 at 91 days. Corresponding relations between the water-cement ratio and the flexural tensile strength have also been determined. The standard deviations when testing the compressive strength of mortars with varying contents of fly ash have been determined. By use of these standard deviations and the relations between the water-cement ratio and the compressive strength, the accuracy of the determination of k has been analysed. The accuracy seems to increase with increasing content of fly ash and in order to obtain accurate values of k, about 30-40 percent by volume of the cement ought to be replaced by fly ash. The activity index has been determined for a number of fly ashes which have been tested at three different laboratories. The value of k for most of the fly ashes was found to be higher than -0,05, 0,15 and 0,5 respectively at 3, 28 and 91 days respectively. These values may be used as limits between acceptable and not acceptable fly ashes. In principle k never should be negative but due to the uncertainty of the testing method it seems to be motivated to chose a value slightly below zero at 3 days.