Abstract
Differentiation of Trypanosoma brucei in the mammal limits the degree of parasitemia and prepares the trypanosome for passage back into the tsetse fly. In an attempt to define the signals that control differentiation, we found that theophylline, in contrast to indomethacin, blocked differentiation, prolonged parasitemia, elevated prostaglandin and cyclic AMP concentrations of rat plasma, and depressed intratrypanosomal cyclic AMP. Relatively nontoxic drugs that alter differentiation are powerful tools for elucidating the events that control this important process.