An enzyme-linked immunoassay for the detection of sulphadimidine in unextracted pig urine is described. Twenty-four hour urine samples from six individually caged pigs, four treated and two controls, were examined during a 10-day treatment and a 12-day withdrawal period. The concentration of sulphadimidine in the urine of the treated pigs increased rapidly after feeding started and decreased on withdrawal. The maximum concentration in a control pig was 308 ng/ml, and this concentration was probably due to contamination of the environment. By the seventh day after withdrawal of the drug its concentration in the urine of the treated pigs was less than 500 ng/ml.