A herd of 116 sows was divided into two groups. Animals in one group had their food supplemented with 1160 micrograms/day of biotin in pregnancy and 2320 micrograms/day in lactation, over a 12-month period. The other group served as a control. Those receiving biotin farrowed significantly more live pigs in parities 2 and 4 than in the control animals; the increase in parity 3 only just failed to reach significance. There was no consistent effect on the weaning to service internal although a significant difference (P = 0.05) was seen with the third litters.