The thesis of this paper is that consumer information influences the size distribution of competing sellers' market shares in a particular manner. By measuring the response of appropriate measures of the distribution of shares to a reduction in the real cost of advertising messages (and thus to an increase in their rate of transmission), it is, in principle, possible to infer whether advertising messages add to or subtract from the stock of information consumers find useful in making product decisions. The principal justification for the research presented here is simply that much of the previous work in this area fails to shed much light on the basic nature of advertising. Statistical associations between, say, the advertising-to-sales ratio and the four-firm concentration ratio are found by some researchers and disputed by others, but there is, typically, residual ambiguity This paper suggests that ignorance has a systematic effect of increasing the market shares of larger brands and reducing those of smaller brands, both relative to a costless information environment. Therefore, cheaper information has an effect of making sellers' shares more equal. To determine whether advertising acts in a manner that is consistent with increases or decreases in this information, the introduction of commercial television in the 1950s provides an appropriate test. The evidence suggests that in markets where the message transmission advantages of television were most prominent-where the real costs of advertising fell the most-the largest sellers systematically lost market share to the smaller.