Various biological mixtures commonly used as microbiological nutrients were added to a chemically defined medium, containing 0.2% (w/v) albumin, in order to assess their ability to support maturation of eggs deposited by adult Schistosoma mansoni in vitro. Of the supplements tested, only lactalbumin hydrolysate and casein hydrolysate stimulated egg deposition and subsequent egg development in vitro. Upon addition of foetal calf serum, further stimulation was noted only in casein hydrolysate-containing media. The addition of mouse red blood cells did not significantly improve the system. Some miracidia which hatched from eggs derived from casein hydrolysate-fortified cultures displayed normal behavioural characteristics, penetrated susceptible snails and established patent infections. These results provide and experimental system for exploring the nutritional requirements of developing miracidia, and the physiological determinants that influence miracidial penetrating capacity and subsequent ability to develop in a susceptible intermediate host.