SummaryA technique for assessing the number, distribution and volume of the lymphoid aggregates (LA) in the guinea pig large bowel is described. Aggregates could be visualised macroscopically after immersion of the bowel in 10% acetic acid and their number and distribution recorded by tracing on plastic sheets. Serial sections through a sample of 13 LA showed that they were ellipsoidal and this was confirmed by mathematical comparison with hypothetical ellipsoids. In practice, only the length and width of each LA could be measured but, using the sample of 13 LA, it was established that regression analysis enabled the volumes of all except the smallest LA to be predicted accurately from the knowledge of length and width alone.