The effect of supplementation with iodine incorporated into biomass of the unicellular alga Chlorella on the content of this element in colostrum and milk was investigated in sows of the Large White breed. Experiments were conducted in two elite herds with different levels of iodine supply in basal feed mixtures. On farm A the feed mixture contained 1 549 µg I/kg, on farm B it was 228 µg I/kg. Pregnant sows on both farms were divided into control and experimental group a fortnight before parturition. Control group comprised 8 sows on farm A and 6 sows on farm B. As the sows on both farms received feed rations of 3 kg feed per head/day, iodine uptake of control animals was 4 647 µg I per head/day on farm A and 684 µg I per head/day on farm B. In addition to this supply experimental sows, 9 animals on farm A and 6 animals on farm B, received 450 µg per head/day of iodine bound in Chlorella biomass. Total iodine uptake was 5 097 µg per head/day on farm A and 1 134 µg per head/day on farm B. Supplementation also continued in the lactation period when total iodine uptake increased with increasing feed consumption. The supplement of organically bound iodine for experimental sows continued to be 450 µg per head/day. Colostrum samples were taken in both groups on the first two days after parturition while milk samples were taken at the end of the third week of lactation. Iodine concentration in these samples and in feed mixtures was determined spectrophotometrically according to Sandell-Kolthoff’s method. Numbers of born and reared piglets, and lactation performance of sows determined by weighing litters on day 21 of piglet life were investigated in individual sows. This litter weight was used to calculate average weight gains of piglets for the period of investigations. The supplementation of Chlorella -bound iodine increased the content of this element in colostrum of experimental sows from 365 ± 81 µg/l to 492 ± 122 µg/l on farm A (p < 0.05) and from 241 ± 70 µg/l to 391 ± 75 µg/l on farm B (p < 0.01). Iodine concentration in milk decreased in all sows, particularly in the animals included in both experimental groups, where it decreased to about half the values detected in colostrum (p < 0.01). Lower iodine content in milk of supplemented sows corresponded with their higher lactation performance. Differences in milk production were obvious mainly on farm B, i.e. in sows with low uptake of dietary iodine. The average lactation performance of supplemented sows on this farm was 61.2 ± 7.95 kg/head while in controls it was 54.9 ± 4.22 kg/head. As the number of experimental animals was low, this difference was below the level of statistical significance. Neither was it possible to prove the higher weight gains of piglets found out in both experimental groups on a significance level. No relationship was established between the number of reared piglets and supplementation of organically bound iodine. It can be concluded from the results that the supplementation of iodine bound in Chlorella biomass increased its concentration in colostrum of sows with both the low and the high dietary uptake of this element, which proved its good utilisation. A possible positive effect of this supplementation on lactation performance of sows and weight gains of piglets should be verified in further experiments.