The distribution of molecular forms of prolactin in serum of women during the 1st postpartum week was studied using column chromatography. Serum was analyzed 1, 4 and 7 days postpartum. Using this technique, three molecular forms of prolactin were detected with approximate molecular weights of < or = 100, 48 and 22 kDa; these were termed 'big-big', 'big' and 'little' prolactin, respectively. The relative amount of each form was measured in relation to the total amount of immunoreactive prolactin eluted from the column. Throughout the first postpartum week big-big prolactin represented a minor fraction of the total immunoreactive prolactin in the serum of lactating women: 4.9%, 5.2% and 4.6% on days 1, 4 and 7, respectively. There was a gradual decrease in the higher molecular weight forms of prolactin (big-big plus big prolactin) from the 1st to the 7th postpartum day. This was associated with a significant increase of the little prolactin from 62.8 +/- 7.2% at the 1st postpartum day to 72.6 +/- 6.5% at the 7th day (p < 0.05).