The kinetics of thermal aggregation of coat protein (CP) of tobacco mosaic virus (TMV) have been studied at 42 and 52 degrees C in a wide range of protein concentrations, [P]0. The kinetics of aggregation were followed by monitoring the increase in the apparent absorbance (A) at 320 nm. At 52 degrees C the kinetic curves may be approximated by the exponential law in the range of TMV CP concentrations from 0.02 to 0.30 mg/ml, the first order rate constant being linearly proportional to [P]0 (50 mM phosphate buffer, pH 8.0). The analogous picture was observed at 42 degrees C in the range of TMV CP concentrations from 0.01 to 0.04 mg/ml (100 mM phosphate buffer, pH 8.0). At higher TMV CP concentrations the time of half-conversion approaches a limiting value with increasing [P]0 and at sufficiently high protein concentrations the kinetic curves fall on a common curve in the coordinates [A/A(lim); t] (t is time and A(lim) is the limiting value of A at t --> infinity). According to a mechanism of aggregation of TMV CP proposed by the authors at rather low protein concentrations the rate of aggregation is limited by the stage of growth of aggregate, which proceeds as a reaction of the pseudo-first order, whereas at rather high protein concentrations the rate-limiting stage is the stage of protein molecule unfolding.