Various parasympathomimetic drugs were injected per anus into female R. appendiculatus, previously fed for 5 days on rabbits, in attempts to induce the ticks to salivate. Pilocarpine nitrate and arecoline hydrobromide gave the best results and these drugs were therefore injected into ticks of various weight ranges and at various concentrations of the drug and the diluent, saline. The drugs were most effective at a concentration of 10% in 6% sodium chloride solution. If ticks salivated, the amount of saliva they produced could be positively correlated with their weight.Sections of salivary glands from 5-day-fed female ticks in the corresponding weight ranges were examined for infection with T. parva. There appeared to be an inverse relationship between the infection rate of the ticks, and the number of infected acini observed, with increase in their weight. It was established that the greatest numbers of infective particles of T. parva should be obtained by the injection of 1 μl of 10% pilocarpine nitrate in 6% sodium chloride solution into ticks weighing between 3.1 and 13.0 mg. Twenty-five infected ticks in this weight range were injected, and the resultant saliva added to concentrated autologous leucocytes in phosphate-buffered saline before injection into a steer. The animal underwent a typical East Coast fever reaction and died, thus indicating that infective particles had been harvested by our methods.Acknowledgements are due to many colleagues for assistance in a variety of ways. Mr M. P. Cunningham, Mr C. G. D. Brown, Dr J. K. H. Wilde, Dr R. Tatchell, Mr M. L. Burdin and Dr G. H. Freeman have given us a great deal of helpful advice. Mr C. Boarer and Mrs D. Backhurst supervised and carried out the processing and staining of the tick sections. Mr B. Wood carried out the weighing of the ticks and prepared the photographs.