A prospective study was performed to find the possible difference in secretory IgA concentration in milk of mothers with term pregnancy labors and those delivering at earlier gestational ages. Since tocolytic drugs and/or glucocorticoid agents are usually given in cases of threatened premature labor, the pre-term group was divided into mothers with or without medication. Thirty two mothers were distributed in three groups: Group I, mothers with preterm labors without any medication; Group II, preterm labors with previous treatment with betamimetics and glucocorticoids; Group III, term labors (see Tab. I). In each of the three groups, three periods were studied: colostral (4 to 5 days postpartum), transitional (8 to 10 days), and mature (14 to 15 days). All mothers were healthy, with good nutritional state, without local inflammation and membranes had been ruptured 12 hours or less before labor. There was no significant difference in the proportion of primiparas and multiparas in both groups. The gestational age was evaluated by amenorrhea and neonatal examination. In all mothers milk was extracted with a vacuum pump to empty the mammary gland. The determinations were made using a specific antibody against the secretory component. The concentration of free secretory component in these milks was practically insignificant. No differences were found in the concentration of secretory IgA among the three groups (Tab. II, Fig. 1) in the periods that were studied, colostral, transitional or mature. The farther away from labor that milk extraction was made, in the periods considered in our study, there is a progressive decrease in the concentration of secretory IgA (Fig. 2).(ABSTRACT TRUNCATED AT 250 WORDS)