Lactoperoxidase activity and thiocyanate content were monitored in the milks of seven individual cows and six goats during lactation. Lactoperoxidase activity exhibited a cyclic pattern with alternating peaks and troughs throughout lactation. Extremely large variations were observed between and within animals. For example, lactoperoxidase activity ranged from 0·05–5·60 U/ml for one cow on different sampling days. Variations between cows were also large, ranging from 0·98–5·1 U/ml on a specific sampling day. Mean lactoperoxidase activity in cows ranged from 1·5–2·7 U/ml with an overall mean of 2·3±1·0 U/ml while the thiocyanate concentration ranged from 6·0–10·2 μg/ml with an overall average of 8·5±5·1 μg/ml. Lactoperoxidase activity means for goats ranged from 0·04–0·16 U/ml with a grand mean of 0·1±0·06 U/ml while the thiocyanate content means ranged from 6·6–8·2 μg/ml with a grand mean of 7·0±2·59 μg/ml. There was no correlation between lactoperoxidase activity and thiocyanate content in either cows' (R2 = 0·011) or goats' (R2 = 0·015) milk. These experiments have revealed that lactoperoxidase activity is affected by many factors including the individual animals, species, feed and stage of lactation. Therefore the exogenous supply of thiocyanate and hydrogen peroxide needed to activate the lactoperoxidase system for raw milk preservation will vary in quantity depending on these factors.