Vitamin auxotrophs of Escherichia coli grown in the presence or absence of the corresponding vitamin were used as substrates for the growth of Dictyostelium discoideum strain Ax-2 in order to investigate some of the growth-factor requirements of this cellular slime mould. Compared with the growth yields observed on vitamin-sufficient auxotrophs and on a prototroph, the yields on lipoate-, folate-, thiamin- or biotin-depleted auxotrophs were low but could be restored by the addition of exogenous vitamin to the slime mould cultures. It is concluded that these four vitamins are essential nutrients for D. discoideum. Similar experiments with other auxotrophs were inconclusive, indicating either that the slime mould does not require pantothenate, nicotinate or vitamin B6 or that the strains of E. coli were insufficiently depleted to detect such requirements. It is also concluded that cobalamin is not an essential nutrient, since the myxamoebae grew well with E. coli which lacks this vitamin when grown in cobalamin-free media. Similar arguments suggest that ubiquinone and vitamin K are also non-essential.