1. A method for preparing rennet reference standards by freeze-drying is described.2. A substrate has also been developed using ordinary spray dried separated milk powder which will give reproducible results under specified conditions. These include:(i) A constant temperature (37° C.) for the duration of the test.(ii) The exclusion of added calcium ions until actual testing begins.(iii) The equilibration of the milk at 37° C. for 2 hr. before use.3. The construction and use of an automatic apparatus for recording rennet coagulation time is also described.4. The instrument error of a single observation is about 1·0%.5. With liquid rennet extracts, the total experimental error, based on one sample of three replicates, is 2% or less: with two samples, it is 1% or less.6. With freeze-dried rennet preparations, the experimental error with two ampoules, sampled thrice with three replicates per sample, was ±2·.0%. With three ampoules it was ±1% or less.