Paczynski1 and Turner et al.2 have suggested that the extraordinary double quasar 1146 + 111B,C found by Hazard et al.3 is a possible gravitational lens. The hypothetic lensing object is unknown. If the lensing object is a cluster of galaxies4 or a cosmic string5,6, it may cause observable effects in the cosmic background radiation. At millimetre wavelengths, a cluster would be expected to cause a reduction in the temperature of the background in the vicinity of the cluster as the result of the Zeldovich-Sunyaev effect. The area of sky on one side of a cosmic string would have a different radiation temperature from that on the other side. If either of these expected signatures were seen in the cosmic background, it would be strong evidence in favour of that hypothesis. We have searched for microwave background inhomogeneities near 1146 + 111B,C. An east-west strip of sky, 16 arc min long and centred on the point between the quasars, was observed with a beam of diameter 105 arc s at 3 mm wavelength. Our observations show only noise with an r.m.s. noise level of 0.0010 K. These observations set limits on the properties of the lensing object if it is a cosmic string or a cluster of galaxies, and we do not confirm either hypothesis.