1. The properties of a recently identified isoform of the human muscle nicotinic acetylcholine receptor (AChR) alpha subunit (alpha +), which in muscle is expressed at similar levels to the alpha subunit, were investigated by both electrophysiological and biochemical approaches following expression in Xenopus laevis oocytes. The single-channel properties of adult (alpha 2 beta delta epsilon) and fetal (alpha 2 beta delta gamma) forms of the human AChR were also investigated. 2. The mean burst duration of adult channels (4.1 +/- 0.3 ms, mean +/- S.E.M., n = 5) is half that of fetal channels (7.9 +/- 0.6 ms, n = 4), while the single-channel conductance is larger (62.2 +/- 0.8 and 37.9 +/- 1.6 pS for adult and fetal channels, respectively), comparable to the developmental changes in single-channel properties observed for other mammalian species. 3. In contrast to the alpha isoform, the alpha + subunit does not bind 125I-labelled alpha-bungarotoxin or monoclonal antibodies directed against the AChR 'main immunogenic region' (MIR), illustrating why the alpha + subunit was first detected through screening of cDNA libraries. 4. By using site-directed mutagenesis to produce subunits that conferred different single-channel conductances on the AChR, we demonstrate that the alpha + isoform is not integrated into functional AChRs. 5. The mutagenesis experiments also revealed that the two alpha subunits within an AChR pentamer are not equivalent within the pore lining region.