Two types of experiments (I and II) were carried out to test the potential of β-1,3/1,6 glucans as immunostimulatory substances in pig feed. In Experiment I, 34 sows were vaccinated against E. coli with 2 ml Nobivac Porcoli (Intervet) 3 weeks pre-partum. The treatment (17 sows) consisted of top dressing the feed daily with 2.5 g MacroGard, 7 days before vaccination until 2 weeks post-partum. No differences were noted in the number of piglets born dead or alive between the 2 groups, but the liveweight of the treated group on day 14 (3.91 kg) was significantly (P < 0.01) lower than in the control group (4.50 kg). In addition, health scores in the control group were better than with the treatment (P < 0.01). Slightly higher titers (non significant) of all determined antigens (K88ab, K88ac, K99, 987P, LT-toxoid) were found in the colostrum of treated sows while in the milk significantly higher titers of K88ab (P < 0.1) and K99 antigens (P < 0.05) were found in the treated sows. A stimulation of the sow's immunoreaction against vaccination seems therefore possible. Retarded recovery from neonatal diarrhoea in the treated group or lower milk production/consumption compared to the control group could have resulted in the poorer performances observed. In Experiment II, 2 x 60 piglets on sow farm I (high infection pressure), weaned at 26 days and 2 x 36 piglets on farm II (low infection pressure), weaned at 25 days, received a weaner diet (Milkivit, Trouw, Belgium) ad libitum supplemented or not with 0.05% MacroGard. On farm I, the weight gain of the piglets in the treated group was significantly (P < 0.05) higher than in the control group – after week 3. Health scores did not differ. Only on day 21 was a significantly lower antibody titer (987P antigen) noted in the serum of the treated piglets. A higher local immunity due to the glucans, resulting in a lower systemic immunity could have resulted in the lower serum titers. On farm II, no differences were noted for daily weight gain, health status or antibody titer level in the serum. In view of the increased antibody titers in the sow milk it seems possible to stimulate immunity in sows and piglets. On farms with high infection pressure, there may be scope to control the disease level by oral administration of natural β-1,3/1,6 glucans.