Recent work on the spectroscopic properties of the B875 (LH1) antenna of Rb. sphaeroides has been performed on the complex purified from membranes of the wild-type solubilised in lithium dodecyl sulphate. In order to facilitate an examination of the properties of the membrane-bound antenna free from other complexes, mutant M2192 was constructed from mutant M21, which contains reaction centres and LH1 but lacks the B800–850 (LH2)complex. This was accomplished by insertion of transposon Tn5 into the puf L gene which encodes the L polypeptide of the reaction centre. This manipulation leaves B875 as the sole pigment protein, which has been confirmed by Southern and Northern hybridisation, gel electrophoresis and fluorescence and absorbance spectroscopy. Evidence from Gaussian deconvolution of the Q y absorbance region, and from fluorescence polarisation, suggests that the long-wavelength species BChl 896 is present, and may be an inherent property of the LH1 antenna.