(_b_) _Action of Baboon Serum._--Contrary to _T. gambiense_, _T. rhodesiense_ is very susceptible to human and baboon sera. Mesnil and Ringenbach[75] showed that a dose of 1 c.c. of baboon (_Papio anubis_) serum cured mice infected with _T. rhodesiense_. In the same dose it acted very feebly on _T. gambiense_. [75] _C.R. Acad. Sci._, 153, p. 1,097. (_c_) _Action of Human Serum._--_1 c.c._ of human serum cured _T. rhodesiense_ mice in three out of four cases; on _T. gambiense_ mice there was no appreciable effect. Laveran and Nattan-Larrier[76] have shown the same, namely, that human sera act on _T. rhodesiense_, but are quite without action on _T. gambiense_. [76] _C.R. Acad. Sci._, 154, p. 18. (_d_) _Trypanolytic Reactions._--Mesnil and Ringenbach[77] have also shown that the sera of animals (man, monkey and guinea-pig) infected with _T. gambiense_ are trypanolytic for the homologous trypanosome, that is, _T. gambiense_, but have no action on the heterologous trypanosome, that is, _T. rhodesiense_. [77] _C.R. Soc. Biol._, lxxi, p. 609. (4) _Cross Immunity Experiments._--(_a_) Mesnil and Ringenbach[78] immunized a monkey (_Macacus rhesus_) against _T. gambiense_. It was inoculated with _T. rhodesiense_ on June 7, 1911; on June 27 trypanosomes appeared, the infection being slight; on July 4 it died. A control died in ten and a half days. [78] _C.R. Soc. Biol._, lxxi, p. 271. (_b_) Laveran[79] immunized a goat and mice against _T. gambiense_. When they had acquired a solid immunity, they were inoculated with _T. rhodesiense_. They became infected like the controls. [79] _Bull. Soc. Path. Exot._, v, pp. 26, 241. (_c_) Laveran and Nattan-Larrier[80] immunized a ram against _T. brucei_, it subsequently became infected with _T. rhodesiense_. [80] _C.R. Acad. Sci._, 154, p. 18. (_d_) Laveran[81] immunized a ram and a sheep against different strains of T_. brucei_. Inoculated with _T. rhodesiense_ they both acquired acute infections and died. Conclusion: _T. rhodesiense_ is not _T. brucei_. [81] _Bull. Soc. Path. Exot._, v, p. 101. When the converse set of experiments is tried, namely, immunizing an animal against _T. rhodesiense_, and then inoculating with _T. gambiense_, the difficulty immediately arises that it is impossible to immunize an animal against _T. rhodesiense_, owing to its virulence. But a partial and transitory immunity to _T. rhodesiense_ can be obtained by treating the infected animal with drugs, such as arsenophenylglycin. The results, so far as they go, seem to show that an animal immunized against _T. rhodesiense_ is immune not only to _T. rhodesiense_, but also to _T. gambiense_, a fact which, according to Mesnil and Léger, does not invalidate the specificity of _T. rhodesiense_, but tends to show that the two trypanosomes are closely related.