Peripheral human blood lymphocytes from healthy blood donors were investigated in vitro to observe the influence of different doses of thymopentin on nonstimulated proliferation, candidin-stimulated proliferation, and phytohemagglutinin (PHA)-induced interleukin 2 (IL2) production. Concentrations of thymopentin ranging from 0.01 to 10,000 ng/ml were used. The proliferation response in non-stimulated cultures was significantly higher in the presence of 0.01, 1, 10, 1,000, and 10,000 ng/ml of thymopentin. There was no significant increase with 0.1 or 100 ng/ml of thymopentin. Thus, three separate peaks were present in these unstimulated cultures: at a concentration of 0.01 ng/ml; between 1 and 10 ng/ml, and between 1,000 and 10,000 ng/ml. These peaks possibly represent three different subpopulations with different sensitivities to different concentrations of thymopentin. Candidin-induced proliferation was significantly higher only at concentrations of 1 and 10 ng/ml of thymopentin, corresponding to the second peak in the unstimulated culture. No other thymopentin concentrations induced significant increase in the candidin-stimulated cultures. No IL2 production was observed in the unstimulated cultures, even in the presence of thymopentin. On the contrary, preincubation with different concentrations of thymopentin influenced PHA-induced IL2 production. A significant increase in the IL2 level was observed in the supernatant of the cultures if 1,000 ng/ml of thymopentin was used in the preculture period. This concentration corresponds to the third peak in the unstimulated cultures. No significant changes were observed with other concentrations of thymopentin. As the measured value of IL2 is a result of a balance between IL2 production and utilization, the above-mentioned findings need further investigation.(ABSTRACT TRUNCATED AT 250 WORDS)