The physiological effects of Pekilo microfungus biomass (Paecilomyces varioti) grown in sulphite spent liquor solution was studied in pigs. Sixteen Finnish landrace gilts weighing about 22 kg were divided into two groups of eight animals each. The animals were fed a barley- and oat-based diet supplemented with vitamins and minerals. The soybean and fish meal mixture used as a protein supplement for the control group was replaced with Pekilo at a level of 13 % of the feed weight in the experimental diet used up till farrowing. The experimental diet used during lactation contained 15 % of Pekilo. The feed consumption and weight gain of the animals were recorded. The experiment lasted almost one year and included the growth, gestation and lactation periods of the sows. The weight gain of the piglets was observed. After weaning of the litter the sows were analysed for blood haemoglobin, haematocrit and white blood cell count, plasma glucose, serum bilirubin, ASAT, ALAT, urea, uric acid, allantoin, total protein and albumin/globulin ratio. The allantoin, uric acid and creatinine contents of a single urine sample were determined quantitatively, and the pH, glucose and protein contents semiquantitatively. The urine density was measured. There were no significant differences between the two groups with regard to weight gain, feed consumption, feed efficiency and reproductive performance. Serum urea and plasma glucose levels were significantly lower in the Pekilo-fed group than in the control group but were within the normal limits for swine. The serum allantoin level and the urine allantoin/creatinine ratio of the Pekilo group were significantly above the control group values. No differences were found between the two groups in the other blood and urine analyses. The use of Pekilo biomass as protein supplement in the diet of breeding sows was found to have no detrimental effects as assessed on the basis of weight gain, feed consumption, feed efficiency, reproductive performance and certain blood and urine parameters.