Maternal growth hormone (GH) levels during pregnancy have been variously reported to be suppressed or, more lately, to be increased. In an attempt to clarify this point, maternal GH levels were estimated with two modern GH polyclonal radio-immunoassays (RIAs) and a new monoclonal enzyme-linked immunoassay (ELISA). The Cambridge Medical Diagnostics RIA and the more specific bioMérieux RIA gave similar results in control non-pregnant patients with raised GH levels, while the monoclonal ELISA gave slightly lower values. The bioMérieux assay gave results about 10 times higher than the Cambridge assay during pregnancy, at 12-61 ng/ml at 16-20 weeks and 47-153 ng/ml at 28-39 weeks (total n = 27). These high 'GH' levels did not correlate with maternal levels of prolactin or human placental lactogen. It is presumed that some unknown GH-like molecule(s) are being estimated in this assay, possibly the recently discovered human chorionic GH. That this is not pituitary GH was confirmed by the monoclonal ELISA, by which GH levels were almost undetectable during pregnancy.