Characteristic curves of first mono-grain layers of photographic materials comprising multi-layers of silver halide grains, which sizes distributed samewise as in observed two typical negative emulsions and all of which necessitated absorption of at least 1, 2, 3, 4 and 10 quanta per grain to form the latentimage ( i ₀) respectively, were calculated. The total number of quanta ranging from 3, 500A to 7, 000A at 1 CMS in NSG type sensitometer was measured and the the unit of log E of above characteristic curves was represented in terms of CMS. From thus obtained characteristic curves of above photographic materials, the maximum speeds of typical negative material were estimated. The results were tabulated in last table.