The purpose of this investigation was to evaluate the effect of a modified cow's milk formula on growth, tolerance and acceptability, which was studied in 2,097 infants selected from those controlled in primary health centers at Santiago, Chile: 1,119 were fed a modified cow's milk formula (LPM), while an unmodified powdered cow's milk (LP) was given to 978 controls of the same age and conditions, as well as solid foods after 4 months of age. Both experimental and control groups were followed along a 4 months period for records of anthropometric data, morbidity, acceptability and tolerance for LP and LPM. We did not find significant differences in growth (values ranging 95 to 100% of NCHS standards W/A or H/A). A slightly higher energy intake and lower protein intake was observed with LPM. Gastrointestinal symptoms were infrequently seen and of similar frequency and character in both 0 to 4 month old groups (4.3% with LPM vs. 5.3% with LP at first control). These gastrointestinal signs were more frequently recorded under LPM than with LP in the 8 to 12 months old group only at first control (colics 8% vs. 4% p < 0.01 and abnormal stools 9.4% vs. 5.5% p < 0.01). Acute acceptability expressed as percentage withdrawal from study was not significantly lower with LPM than LP (5 to 15% vs. 2% N.S.). The proposed modified formula seems to be a good alternative to powdered cow's milk for infants.