Moxalactam exists in two epimeric forms, R and S. The in vitro activity of these two epimers was compared with that of material available for clinical and laboratory use (R + S moxalactam). Generally, R moxalactam was twice as active as the S form. The stability of R + S moxalactam was studied at 37, 20, 4 and -20 degrees C in buffer and serum. Only in serum at 37 degrees C was there any appreciable loss of activity (half-life, 8 h). The stability of R and S epimers was studied separately, and the composition of the resulting equilibrium was investigated. At 37 degrees C in serum, one-half of the excess of either R or S over the equilibrium mixture was converted into the equilibrium mixture in 1.5 h. The proportions of R to S in an equilibrium mixture in buffer were 50:50, but in serum they were 45:55. It is doubtful whether these differences in stability and activity will have any significant clinical importance.