In order to determine the undesirable side effects of standard commercially available plasma substitutes, 450 stationary patients covering 5 age groups were randomly allocated to various methods of pre-medication and 3 batches of modified fluid gelatin (Neo-Plasmagel). The incidence of allergoid and anaphylactoid reactions depended on the pre-medication (p less than 0.025). Atosil proved to be an strong histamine blocker: the incidence of reactions between the control and the promethazine group was p less than 0.005. Seen as a whole reactions among females were greater than with males (p less than 0.025). No relationship could be determined statistically between the different batches and the incidence of side-effects. -The prophylactic use of Neo-Plasmagel in orthopaedic patients is therefore dependent on a sufficient blockade of histamine receptors.