Lysozyme concentrations and bacterial colony counts were determined in 399 human milk samples obtained from 42 mothers (18 mothers of term infants, 24 mothers of preterm or small-for-date neonates). The average lysozyme concentration was 21.39 +/- 13.19 mg/L. Lysozyme concentrations were significantly greater in preterm (24.99 +/- 15.05 mg/L) than in term milk (14.89 +/- 9.83 mg/L) (p less than 0.05). Bacterial colony counts did not decline with increasing lysozyme concentration, i.e., no correlation between lysozyme concentration and bacterial count was found. Despite this lack of correlation, a significant correlation between lysozyme concentration and rate of weight gain was observed: the higher the lysozyme concentration, the better the weight gain. The mechanism of this observation remains to be elucidated; a trophic effect of lysozyme is suggested as a possible mechanism.