Biochemical and electron microscopic analyses of heat-shocked suspension cultures of Peruvian tomato (Lycopersicon peruvianum) revealed that a considerable part of the dominant small heat shock proteins (hsps) with an Mr of approximately 17,000 are structural proteins of newly forming granular aggregates in the cytoplasm (heat shock granules), whose formation strictly depends on heat shock conditions (37 to 40 degrees C) and the presence or simultaneous synthesis of hsps. However, under certain conditions, e.g., in preinduced cultures maintained at 25 degrees C, hsps also accumulate as soluble proteins without concomitant assembly of heat shock granules. Similar heat shock-induced cytoplasmic aggregates were also observed in other cell cultures and heat-shocked tomato leaves and corn coleoptiles.