After 24 h desiccation at 28–29 °C and approximately 50 % relative humidity about 70 % of B. microplus larvae drank sufficient solution containing 1·5 mCi of 32P per ml for their movements on the host to be followed with a portable G-M counter. Reduced viability of the labelled ticks could not be detected when they were kept for 24 h on the host and either allowed to feed or prevented from doing so. When feeding was continued for 72 h or when larvae were incubated for 3–4 weeks, adverse effects were found, the severity of which could not be directly related to the amount of 32P label. Desiccation, handling of larvae and exposure to radiation in the drinking solution may have contributed to their reduced viability.

Since resistance to the tick is manifest within the first 24 h after infestation labelled larvae could be used to compare development on different hosts during this time. Even after feeding for 72 h, larvae were more adversely affected by host resistance than by labelling damage.

We would like to thank Mr C. A. Schuntner for helpful advice and Mrs J.Galligan for technical assistance. The financial support of the Australian Meat Research Committee is gratefully acknowledged. Messrs D. Hancox and S. R.Henderson assisted with some of the experiments.