The objective of this study was to evaluate the suspected endocrine disruptive effect of molasses included in cattle feed. During the mid 1990's a potentially serious, chronic syndrome was reported in well-managed beef and dairy herds from unrelated parts of South Africa. Farmers reported that it manifested as various combinations of seriously decreased production, increased reproductive disorders, apparent immune incompetence in previously immune competent animals, various mineral imbalances in non¬deficient areas and goitre, perceptible by enlarged thyroids. Farmers related this syndrome to certain batches of sugarcane molasses and molasses¬based products. Their opinion was based on observations that dramatic improvements in health and productivity often followed the withdrawal of the molasses component of the diet. The syndrome had all the characteristics of an "endocrine disruptive syndrome". Endocrine disrupting compounds (EDCs) are exogenous substances that have the potential to alter the functions of the endocrine system and consequently cause ill health. The EDCs comprise a diverse group of compounds that are widely used in detergents, in the paint and plastics industries and as wetting agents in agricultural remedies. Contamination of molasses with these compounds could occur as a result of uptake of chemicals by growing sugarcane or by the use of contaminated river water in the extraction of sugar from chopped sugarcane. It was essential to investigate the safety of molasses, currently available as stock feed in South Africa and the rest of the world, to allay any fears created by the farmers. Four batches of molasses previously collected from four different sugar mills located in different parts of the country were screened for potential endocrine disruptive activity, including oestrogen and thyroid activity and immune suppression. Two batches of molasses were selected to be used in a calf feeding trial. Thirty-two, 4 to 6 week-old Holstein bull calves of approximately the same weight, and supplied by the same rearing facility, were included in a single phase, three treatment, parallel design experiment. Calves from the three experimental groups were housed in the same facilities, under the same conditions and fed diets with similar composition. Two of the groups had molasses (Umzimkulu and Komatipoort) included in their rations and the control group was fed a ration where no molasses was added, but fermentable sugars and minerals were included. The weight gain of the calves was recorded over a six-month study period. Regular clinical examinations were conducted and clinical pathology parameters, immune responses and endocrine effects were evaluated. Even though endocrine disrupting effects were detected in in vitro screening tests, these could not be reproduced in calves under experimental conditions. The two batches of molasses utilized in the calf feeding trial did not induce major differences in any of the parameters measured, except a lower weight gain in the Umzimkulu molasses fed group which tended towards significance. It appears improbable that these two batches of molasses had any endocrine disruptive or immunosuppressive effects. Therefore, these two batches of molasses per se were not deleterious when fed to calves. / Dissertation (MSc (Veterinary Sciences))--University of Pretoria, 2006. / Paraclinical Sciences / unrestricted