the natural logarithms of (mut - AUt+at) against time (t) is a straight line. The resulting slope (units, s -l) is the observed first order rate constant (kob~); the second order rate constant (k; units, M -1 s -1) is calculated from kobs = k x [NaC1]. The hemolysis time is the time required for the initial arbitrary units to fall to half the initial value. Results With the methodology described above for following the hemolysis rate constant there is no light absorption by a suspension of red blood cells in NaCl hypotonic solutions at 690 nm and the method is based on the principle that turbidity variation is directly related to particle intactness, which is measured by the intensity of the transmitted light (inversely related to scattering effects). The values of the observed first order rate constant and of hemolysis time are obtained from results in the absence and presence of dipyridamole. Red blood cell shapes are classified and the influence of dipyridamole concentrations assessed and related to the changes in the kinetic parameters. Conclusions The experiment outlined above is inexpensive because the materials utilized are readily available, is easily conducted by medical and pharmaceutical students or post-graduate students, and deals with enzyme kinetic concepts and makes it possible to investigate red cell volumes and their shapes. As red blood cells in vitro do not behave as an "end product" but, instead, the metabolic pathways and ionic pumps still operate, this practical exercise could be modified by changing the suspension medium for example by the addition of glucose, adenine or a calcium ionophore. One of the didactic advantages of this laboratory class experiment is to raise the curiosity of students to questions related of side-effects caused by drugs. In an important sense "how does it work?" is a good question. The choice of dipyridamole was based on its molecular structure (it is a large molecule with four hydroxyl groups) and on some reported properties, such as inhibition of RBC lithium and anion transport 7 and, as mentioned above, this drug induces discocyte --> echinocyte transformation. Whether RBC echinocytes induced by dipyridamole act as an activator of platelet disaggregation is an open question. Other echinocytogenic and stomatocytogenic compounds could likewise be introduced, according to class time available and the ratio between number of tutors and students.