The use of biological feed additives in animal production systems is becoming widespread, largely in response to consumer pressure and legislation concerning the use of antibiotics and hormone implants. Howewver, published data on their efficacy are limited. Yea-sacc,a high strength yeast culture, is one such product. A yeast-culture is a dry product composed of yeast and the media on which it was grown, dried in such a manner as to preserve the fermenting capacity of the yeast. The yeast used in Yea-sacc is a selected strain of Saccharomyces cerevisiae, and the yeast culture is produced using a high density fermentation technique. A trial was conducted to examine the effect of including Yea-sacc in the diet, on the performance of early-weaned calves.