T-cell growth factor (TCGF) activity was studied in phytohemagglutinin (PHA)-stimulated peripheral blood mononuclear cells (PBMC) from 10 type-1 diabetic patients who had been diagnosed within the previous 12 months (group A), from 9 diabetic patients in whom the duration of disease was more than 1 year (group B) and from 12 healthy controls (group C). The effects of indomethacin on PHA-induced TCGF activity and the effects of adherent cells (macrophages) from group A and group C on TCGF production of normal group-matched non-adherent cells (lymphocytes) were also studied. TCGF activity was assayed on TCGF-dependent blast cells and calculated as a stimulation index (SI). TCGF activity in group A (SI 0.86 +/- 0.8) was significantly different from that in group B (SI 1.75 +/- 1.02; P = 0.037) and in group C (SI 1.91 +/- 1.29; P = 0.023). Following the addition of indomethacin, TCGF SI was 1.35 +/- 0.74 in group A, 1.85 +/- 0.73 in group B and 2.06 +/- 1.19 in group C. The responses to indomethacin were found to correlate with the basal TCGF activity in all subjects (r = -0.48; P = 0.006) independently of the disease process studied or its duration. No correlation was found between TCGF activity and parameters of metabolic control (HBA1c and fructosamine). Interestingly, a significant inverse correlation was found between TCGF activity and the required dose of insulin only in group A (r = -0.66; P < 0.05). Adherent cells from diabetic patients were found not to inhibit TCGF production.(ABSTRACT TRUNCATED AT 250 WORDS)