The motivation of this discussion is threefold: to integrate transaction costs (TCs) into a standard model of the firm; to examine the interaction between organisational factors (i.e. TCs) and standard demand-cost factors; and to analyse key propositions of transaction cost economics with the general model. Two sets of results are derived. First, when analysis is based on significant interaction between organisational effort and production costs two possible organisational solutions can exist. First we have a “normal” relationship that the existence of small firms is subject to a threshold effect for transaction complexity. Secondly large firms can develop because of interactions between organisation effort and marketing and production cost...