This study investigates the heterogeneity in the synthesis of heat shock proteins (hsp) in normal human lymphocytes and leukaemic cells. The results show that leukaemic cells exhibit different patterns of hsp synthesis compared to normal lymphocytes. Normal lymphocytes rapidly induce major hsp (hsp90, hsc70, hsp70) upon heat stress. In contrast, leukaemic cells showed three distinct patterns of hsp induction: (a) all three major hsp are heat-inducible, similar to normal lymphocytes; (b) hsp90 and hsp70 are inducible, while hsc70 is not; and (c) hsp90 and hsp70 are inducible, with a significant amount of constitutive hsc90. The synthetic patterns of hsp in leukaemic cells from patients and cell lines are summarized in Table I. The expression of hsp is partially dependent on the differentiation stage in certain cell lineages. Differentiation of HL-60 cells was accompanied by increased inducibility of hsc70. The study suggests that the degree of maturation of leukaemic cells influences hsc70 inducibility. Additionally, some leukaemic cells constitutively synthesize a large amount of hsc90, which is not affected by heat shock. The presence of abundant hsc90 may affect the proliferative capacity of leukaemic cells. The findings indicate that the synthesis of hsp in leukaemic cells is heterogeneous and may be influenced by the differentiation stage and maturation level of the cells.This study investigates the heterogeneity in the synthesis of heat shock proteins (hsp) in normal human lymphocytes and leukaemic cells. The results show that leukaemic cells exhibit different patterns of hsp synthesis compared to normal lymphocytes. Normal lymphocytes rapidly induce major hsp (hsp90, hsc70, hsp70) upon heat stress. In contrast, leukaemic cells showed three distinct patterns of hsp induction: (a) all three major hsp are heat-inducible, similar to normal lymphocytes; (b) hsp90 and hsp70 are inducible, while hsc70 is not; and (c) hsp90 and hsp70 are inducible, with a significant amount of constitutive hsc90. The synthetic patterns of hsp in leukaemic cells from patients and cell lines are summarized in Table I. The expression of hsp is partially dependent on the differentiation stage in certain cell lineages. Differentiation of HL-60 cells was accompanied by increased inducibility of hsc70. The study suggests that the degree of maturation of leukaemic cells influences hsc70 inducibility. Additionally, some leukaemic cells constitutively synthesize a large amount of hsc90, which is not affected by heat shock. The presence of abundant hsc90 may affect the proliferative capacity of leukaemic cells. The findings indicate that the synthesis of hsp in leukaemic cells is heterogeneous and may be influenced by the differentiation stage and maturation level of the cells.