In this work, a mathematical model incorporating the delay on the batch penicillin production is proposed aiming to better describe the temporal antibiotic concentration profile. By introducing a delay time parameter into the original model, it was possible to improve the antibiotic formation dynamics description, mainly at the beginning of the bioprocess, when the original model is fault. From the proposed model was possible to estimate the duration of the delay in the penicillin production as being 37 hours. Parameters and models were validated by adequate statistical tests.