The aim of this project was to examine the process by which cultured cells of Papaver somniferum and related species, biotransform codeinone to codeine. It was confirmed in this study that plants of P.somniferum produce the morphinan alkaloids thebaine, codeine and morphine and therefore, possess the enzyme involved in the biotransformation of codeinone to codeine. However plants of P.bracteatum and P.orientale only accumulate thebaine apparently lacking one or more of the enzymes necessary to convert thebaine to morphine. Suspension cultures derived from P.somniferum, P.bracteatum and P.orientale all produced the morphinan alkaloid(s) common to the parent plant but in significantly lower amounts. P.somniferum suspension cultures converted a maximum of 35.3% of added codeinone to codeine. However, 'meristemoid' cultures of P.somniferum, which displayed a high state of differentiation and consisted of densely cytoplasmic cells organised in recognisable morphological structure, converted up to 60.0% of added codeinone to codeine. By contrast, suspended cells of P.bracteatum and P.orientale biotransformed less than 5% of added codeinone to codene. This strongly suggests the existence of a relationship between biotransformation capacity (amount of product produced per specified volume or unit of biomass) and culture origin. Immobilisation increased the biotransformation capacity of P.somniferum suspension cultures but not of P.somniferum 'meristemoid' cultures. Suspension cultures of P.somniferum, at different stages of development, converted approximately equal quantities of codeinone to codeine over a 72h period. However, the rate at which codeine was produced in these cultures varied considerably and was fastest in cultures entering the stationary phase. This also corresponded to the stage at which the concentration of endogenous alkaloids in the cells was highest.