A quantitative baiting assay system for Phytophthora cinnamomi with greater sensitivity than the orthodox serial dilution end-point method was developed. A system of efficiently handling large numbers of subsamples using racks of tubes is described, and the factors determining baiting efficiency with blue lupin (Lupinus angustifolius) radicles were studied. Significantly greater baiting efficiency was obtained when the subsample size was decreased. The optimum temperature was found to be ≈ 25°C for baiting and 20–25°C for bait incubation. Air-drying of soil for two days killed P. cinnamomi propagules. Escape of zoospores from test soil was reduced by the presence of overlying material. Reduction was greatest when the overlying material was of fine particle size and of greater depth. This confirmed the need in assay work to keep soil subsample size small and avoid destruction of the soil structure in preparation. The peak of zoospore release from naturally infested soil occurred on the first day. Infection on radicles decreased as a function of distance from radicle tip.