Abstract: I n t r o d u c t i o n Northern blot and dot/slot blot analyses are the most frequently used methods for assaying the levels of specific polyadenylated messenger RNAs in plants. We have tried to use both of these methods to examine the level of expression of a gene for the small subunit (SSU) of soybean ribulose-l,5-bisphosphate carboxylase/oxygenase in a large number of transgenic plants. Both methods were found to be unsatisfactory with total RNA preparations. We encountered problems caused by hybridization of the probe for the introduced gene to rRNA and by unquantitative transfer onto nitrocellulose. This produced an unacceptable levelof error when m R N A levels were estimated from band densities on autoradiographs. We decided that separation ofpolyA + RNA would give greater sensitivity. However, because we were interested in screening over 100 transgenic plants, separation on oligodT cellulose was too time-consuming. We found that separation of polyA + R NA using hybond-mAP (Wreschner and Herzberg, 1984), a messenger-affinity paper (Amersham International plc, Amersham Laboratories, Whi te Lion Road, Amersham, Buckingham, England), followed by hybridization to specific probes, is a rapid, quantitative and reproducible method that allows large populations of plants to be easily screened.