Abstract When haploid cells of Physarum flavicomum reach the end of the growth phase in liquid semidefined medium they differentiate to dormant microcysts. Several compounds, effective as proteinase inhibitors in cell extracts, either blocked or delayed microcyst formation in semidefined medium. In addition, the compounds (except chloroquine) affected the conversion of swarm cells to myxamoebae. N-α- p-Tosyl- l-lysine chloromethyl ketone blocked microcyst formation and the vast majority of the cells existed as swarm cells. Chloroquine was an effective inhibitor in that the cells quickly became inactive; only about one-third of the cells converted to microcysts while the remaining cells were abnormally small. Both antipain and leupeptin inhibited normal microcyst formation and those microcysts produced were morphologically aberrant. Microcysts of normal appearance were produced in the presence of pepstatin and hemin but the process was significantly delayed.