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There are a number of factors concerned in the combination of antitoxin with crude toxin and toxoid contained in toxic culture filtrates. These factors must be considered before theories are advanced to account for any observed phenomena.The object of this paper is to catalogue some of these factors, to give such experimental evidence as we have accumulated and to give a few instances from the literature where these factors have been ignored. The chief factors are as follows. Antitoxin has a greater affinity for toxin than for toxoid either naturally produced or formed by the action of formaldehyde on toxin.Combination between antitoxin and toxin or toxoid becomes firmer with time, this effect is more pronounced with toxoid or formol toxoid than with toxin.The relative affinity of formol toxoid for antitoxin compared with that of toxin varies with different preparations and can be correlated to some extent with immunizing efficiency. Barr & Glenny (1949) described a method for measuring affinity of toxoid for antitoxin in terms of an 'affinity coefficient'.The 'differential region' of Ehrlich, which is the difference between Lo and L + doses, varies in extent depending upon the relative amounts of toxoid and toxin in a toxic filtrate and can be correlated with the number of LD5.

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