The phospholipid composition of amniotic fluid has been examined in 30 twin pregnancies with a mean gestation of 37.7 weeks. Before the onset of labour there was no difference in ratios of lecithin (L), phosphatidylinositol (PI), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) to sphingomyelin (S) between the first and second twin. With the start of contractions (6 patients) there was a significant increase in the L/S and PE/S ratios in the first twin (L/S ratio twin I 10.8 +/- 5.2, twin II 7.0 +/- 3.8; PE/S ratio twin I 0.7 +/- 0.2, twin II 0.4 +/- 0.1). Growth retardation of one fetus did not alter the phospholipid profile in the amniotic fluid. Amniotic fluid concentrations of glucocorticoids, measured before and during labour in both twins by specific immunoassay, were the same in both sacs before labour. During labour concentrations of unconjugated and conjugated glucocorticoids were significantly increased in the first sac (twin I unconjugated 113.2 +/- 16.6, conjugated 505.1 +/- 115.9; twin II unconjugated 69.5 +/- 41.4, conjugated 284.3 +/- 124 nmol/l). Amniotic fluid from growth retarded fetuses did not show higher glucocorticoid levels. In subsequent studies the glucocorticoids were separated by chromatography and only cortisol reacted significantly with the antibody used under the conditions of the assay.