Langley was aware of the similarity between his concept of receptive substances in cells and Paul Ehrlich's side-chain theory. However, he maintained that they had uncovered somewhat different phenomena. Both assumed, as he put it, ‘atom-groups of the protoplasm’ of the cell, to which substances could bind. But while Ehrlich's side-chains were ‘fundamental’ to the cell's life (i.e. the cell would die if they were all occupied by poisons), Langley's ‘receptive substance’ merely modified the cell's function when a drug or hormone bound to it. Significantly, Langley never adopted Ehrlich's more general term ‘receptor’, but stuck to his own term ‘receptive substances’ throughout his life. Ehrlich, on the other hand, conceded that the receptor concept was also applicable to drugs and medicines, not only to toxins or foodstuffs.