Recombinant human interleukin-3 (rhIL-3) production by a Streptomyces lividans strain is subject to severe proteolytic degradation. Human interleukin-3 (rhIL-3) concentration in the culture supernatant typically reaches 100-120 mg l(-1) about 20 h after inoculation, but the titer drops rapidly as the result of proteolysis. Reducing the culture temperature at this point has a dramatic effect on product yield, increasing the final concentration in the supernatant at least two-fold.