“…Isomeric monomethylbenz[a]anthracenes (MBA's) encompass a wide range of carcinogenic activities (Wislocki, Fiorentini, Fu, Yang & Lu, 1982). They, and the dimethylbenz[a]anthracenes (DMBA's), vary in shape from the appreciably distorted or type II (Briant, Jones & Shaw, 1985) when either or each of the bay sites 1 and 12 carries a substituent (Jones & Shaw, 1987), to the almost planar, with methyl substituted at a non-bay site as in 11-MBA (Briant & Jones, 1984). 5-MBA is one of the less carcinogenically active MBA's and determination of its crystal and molecular structure is of especial interest in that substitution is at the phenanthrenic K region, directly opposite the bay.…”