The effect of colostrum of mothers of preterm and full-term newborns on the phagocytic activity for latex particles by normal peripheral blood polymorphonuclear cells was examined. The results showed that human colostrum contains a phagocytosis-promoting factor(s), which not only increases the number of phagocytic cells, but also stimulates the phagocytic activity of the individual cell. This factor(s) was more active in preterm colostrum. Thus, while term colostrum increased the number of phagocytic cells by 95% in comparison with the control, preterm colostrum enhanced the number of phagocytic cells by 300%. Similarly, term colostrum increased the phagocytic capacity of the individual cell by 51%, whereas preterm colostrum by 137%. The difference between the results in all experimental points was statistically highly significant.