Large dairy plants must control starter activity to maintain the processing schedule associated to the multiple use of vats. Although bacteriophages are an important inhibitor of starter activity, bulk starter preparation procedure is also an important factor which influences the subsequent activity of the lactic acid bacteria. The aim of this article is to discuss how the various steps in the starter preparation procedure may influence the population, strain ratio and acidifying activity of milk-grown bulk starters. Aspects covered are faulty gauges or analysis equipment; milk powder selection; milk solids; heat treatment applied to starter milk; chemical contaminants in the milk; inoculation level; incubation temperature; incubation time; cooling of the bulk starter; stratification of starter solids upon storage; reference cultures; and errors made during use of the starter itself.