Seven alkyl and aryl homologues of O-methylsterigmatocystin (OMST) were synthesised and fed in separate experiments to a mutant of Aspergillus parasiticus capable of converting sterigmatocystin (ST) to aflatoxin B1 (AFB1). Their conversion to AFB1 was followed over a time period and it was found that O-propylsterigmatocystin (OPRST) was converted to AFB1 more rapidly than O-ethylsterigmatocystin (OEST) or OMST or ST itself. The aryl derivative O-benzoylsterigmatocystin (OBzST) was converted at the slowest rate. These results show that alkyl and aryl homologues of OMST may be converted to AFB1, suggesting that the methylation of ST is not an absolute requirement for its conversion to AFB1. It seems likely that whatever enzyme(s) are involved in this process exhibit relative specificity. As to whether alkylation of ST is an obligatory step in AFB1 biosynthesis is neither supported nor disproved as the fungal cells used are presumably capable of methylating ST. The fact that the propyl derivative showed fastest conversion is not necessarily significant as this may be due to faster diffusion of the least polar of the derivatives through the cell membrane.