A bioequivalence study was performed in 12 young cattle in order to compare 2 injectable solutions of spiramycin containing 60 M IU/100 ml (Suanovil 20) and 100 M IU/100 ml, respectively. In a cross-over design, a single dose of 100,000 IU/kg bw was administered intramuscularly, allowing 8 d between the 2 administrations. For each formulation, the following parameters were determined: maximum serum concentration (Cmax) and time to reach peak concentration (Tmax), mean residence time (MRT), area under the time-concentration curve (AUC), half-life (t1/2 beta). Statistical analysis of t1/2 beta, Tmax (Wilcoxon's test) and Cmax, MRT, AUC (ANOVA), indicated no significant differnces between the two formulations (P greater than 0.05). Westlake's confidence intervals calculated for a 95% probability were 23.1% for Cmax, 11.1% for MRT and 13.2% for AUC, respectively, which confirmed the bioequivalence of the 2 formulations. It must be noted that an experimental design using 12 animals is generally insufficient to demonstrate bioequivalence, according to the recommended statistical criteria.