Lactobacillus rhamnosus MA27/6B and L. acidophilus MA27/6R are strains used in feed as probiotics. Their safety profiles and growth-stimulating properties were investigated via in vivo studies on young Swiss mice. After repeated administrations of different probiotic preparations in drinking water, safety parameters determined from liver, spleen and total weight remained unchanged. The growth-stimulating properties of viable or dead lactobacilli were studied after supplementation in drinking water. The feed intake (FI), water intake and body weight gain (WG) of the animals were compared to those of control mice. The lactobacilli supplementation of a sub-optimal diet made of barley allow recording of measurable growth performance of mice. It significantly increased WG compared to control groups (P < 0.01), by +28.9% and +31.7% for L. rhamnosus MA27/6B and L. acidophilus MA27/6R, respectively. This WG was correlated with a decrease in the consumption index. The effect of the dose ingested was also investigated: 10(8) lactobacilli CFU/mouse/day produced greater WG than 10(2), 10(4) or 10(6) CFU/mouse/day. No significant differences in growth performance parameters were observed between mice fed with 10(8) cells of viable or nonviable preparations. The mouse assay described could be used as a preliminary criterion when screening candidate probiotics for growth performance properties.