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SUMMARY : Developmental forms of Trypanosoma ldsi continue to increase in number when the 'crisis' (normally occurring at the loth day) is inhibited; the inclu- sions which they contain also increase in number, density and refractile power. Suspensions of killed developmental forms containing multiple inclusions have greater protective power against a challenge infection than have similar suspensions of adult forms which contain a single inclusion. It is suggested that the multiple inclusions contain an antigenic factor which stimulates the production of a try- panocidal antibody and reacts with it to produce the crisis, but the single inclusions of the adult forms which survive the crisis have different antigenic properties. The formation of antibody in the early stages of an infection of the rat Rattw norvegicus with Typanosoma lewisi has long been recognized as a con- stant phenomenon, and to be related to the 'crisis' which occurs at about the loth day of the infection, checks the rise in parasitaemia and produces an abrupt fall (Steffan, 1921). This crisis marks the end of the initial 'poly- morphic ' phase of the infection, leaving only ' monomorphic ' trypanosomes in the blood; these survive for a period varying from several days to several months. The final elimination of this monomorphic infection is also due to the formation of an antibody, but this has been shown (Taliaferro, Pizzi & D'Alesandro, 1958) to be distinct from antibody produced in the early stages of the infection and will not be considered further in this paper. This paper deals with the relationship of the multiple inclusions which appear in the posterior end of the dividing forms of Trypanosoma lerrmki to the formation of antibody which occurs at the time of the crisis; the suggestion that there might be such a relationship arose from the appearance of these inclusions at the time of the crisis with T. Zewisi and from a similar finding in the very different conditions of an infection of the rat with 2'. rhodesiense (Ormerod, 1958). The work consisted of three experiments, details of which will be given separately in the sections on Methods and Results : (1) A study of the effect of serum, obtained from infected rats in which a normal crisis had occurred, on the number, density and refractile power of inclusions. (2) A similar study of inclusions in rats in which the crisis had been in-

A Study of Cytoplasmic Inclusions in Trypanosoma lewisi and their Relationship to the Formation of Antibody