A defined salts medium containing lactose and MnSO4 as selective agents is described. This medium inhibits nearly all organisms of the soil microflora except Agrobacterium spp. against which it has no repressive or mutagenic effect. By using a modified test for 3‐ketolactose production together with plant inoculation, qualitative and quantitative assays for agrobacteria are possible. Copyright © 1969, Wiley Blackwell. All rights reserved