The in vitro inhibitory effect of macrolides, synergestines and lincosamides on Toxoplasma gondii was evaluated using an enzyme immunoassay performed directly on the infected cultures. Antimicrobial agents were tested at various concentrations, and regression models were used to quantify the relationship between optical density values (correlated to Toxoplasma count in the cultures) and drug concentrations in the cultures. A significant inhibitory effect was observed with the macrolides (erythromycin, roxithromycin, midecamycin, spiramycin, josamycin and oleandomycin) for concentration of 0.1 to 4 micrograms/ml. This effect progressively increased with increasing concentration of drug in the cultures and reached its maximum value for concentrations of 40 to 250 micrograms/ml. Similarly, clindamycin was found to have a significant inhibitory effect for concentration ranging from 0.5 ng/ml to 100 micrograms/ml. Only a partial inhibitory effect was noted with virginiamycin, pristinamycin and lincomycin. The concentrations giving a 50% inhibition of Toxoplasma growth in the culture were determined from regression models and ranged between 0.15 and 12 micrograms/ml according to the drug. This results confirm that macrolides, lincosamides are effective against Toxoplasma gondii in vitro, and can be proposed for treatment of toxoplasmosis because of their large diffusion, their good tolerance and their concentration in tissues such as lung and placenta.