Assuming an irreversible, random, end-to-end aggregation of a small percentage of potato X virus in tobacco sap, the flow birefringence parameters of such polydisperse systems have been calculated. The extinction angles χ were evaluated for different values of the ratio G/θ1(G gradient θ1, rotary diffusion constant of non-aggregated virus) for preparations containing between 0 and 15% by weight of aggregated material. A particle length of 6200 Å for the non-aggregated virus particle was derived and an amount of 13–15% aggregated material was estimated.A partially purified and fractionated preparation of potato X virus was also investigated and an average particle length of 5400 Å was derived. Because of the possible contribution of small fragments, this length was taken as the lower limit.The flow birefringence – gradient curve indicated that the potato X virus behaves like a rigid particle. No evidence of flexibility was obtained from extinction angle – gradient measurements in the range 0.1–0 M phosphate buffer concentrations at pH 7.2.

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