Sixty piglets selected after weaning at 4 weeks of age were assigned to five replicates of twelve animals each. In each of these replicates the postprandial variations in serum pteroylglutamate after the ingestion of twelve sources of dietary pteroylglutamic acid were recorded twice weekly at 10 and 16 weeks of age. In six of these sources of pteroylglutamic acid the chemically pure form of the vitamin was incorporated into a semi-purified diet at concentrations varying between 0 and 1·0 mg/kg. The six other sources were provided by a soya-bean meal, rapeseed meal, maize, barley, wheat, and a commercial vitamin premix. The concentrations of pteroylglutamates measured by radioimmunoassay in the different feedstuffs were, in most cases, far from the values reported in the literature, except for maize. Indeed, while total pteroylglutamates in barley, wheat and rapeseed meal were lower by 35–56%. 17–50% and 60% respectively compared with references values, the corresponding values for soya-bean meal ranged from one third to twice as much. The area under the curve (AUC) of the pre- and postprandial (1, 2, 3, 5 and 7 h) serum pteroylglutamate following ingestion of increasing levels of chemically pure pteroylmono- glutamic acid was used to derive a regression for the 100% bioavailability of dietary pteroylglutamic acid. The corresponding AUC for the feedstuff sources of pteroylglutamates were used in the regression to determine the proportion of bioavailable pteroylglutamates out of total pteroylglutamates measured in these ingredients. No relationship (P 0·66) was found between the level of chemically pure dietary pteroylmonoglutamic acid and the postprandial AUC. In fact, there was no significant (P 0·11) increase in the postprandial concentration of serum pteroylglutamate for any of the pteroylglutamate sources used except for wheat. Moreover, values tended (P 0·08) to be lower at 5 and 7 h postfeeding except for wheat and barley. It was hypothesized that this decrease is probably linked to the postfeeding variation in bile secretion which drains considerable amounts of circulatory pteroylglutamates. The results of the present experiment indicate that further research on analytical procedure is needed in order to provide a reliable method for measuring concentrations of pteroylglutamic acid in different sources of a given feedstuff used in pig feeding. In addition to this analytical concern, the measurement of the proportion of bioavailable pteroylglutamic acid in feedstuffs for pigs using postprandial variations of serum pteroylglutamates appears to be technically hazardous.