Background: For more than 50 years, antibiotics have been used to maintain animal welfare and improve efficiency. Recently, antibiotics were found in the muscle, liver, and kidney of guinea pig carcasses put up for sale and human consumption, which is a public health issue. Probiotics are supplements of live microorganisms that, when administered in adequate doses, could replace growth-promoting antibiotics. Aim: This study analyzed the effect of the administration of an oral probiotic mixture on the guinea pigs productive performance (Cavia porcellus). Methods: Fifty male guinea pigs, weaned at 14 days of age, were distributed in a completely randomized design of five treatments with ten repetitions for each group. The treatments were CONTROL group without probiotic; PROB 1 given 1 ml of probiotic; PROB 2 with 2 ml of probiotic; PROB 3 with 3 ml of probiotic; and antibiotic growth promoter (AGP) was given 300 ppm zinc bacitracin. The microorganisms used in the probiotic were Enterococcus hirae, Lactobacillus reuteri, Lactobacillus frumenti, Lactobacillus johnsoni, Streptococcus thoraltensis, and Bacillus pumilus. Productive parameters were evaluated from weaning to 70 days of age. Results: No statistically significant difference was found between the treatments on forage dry matter intake (DMI), concentrateconcentrate DMI, or total concentrate DMI (p > 0.05). Similarly, no statistical difference was found between the treatments in terms of final weight or weight gain (p > 0.05). Regarding the feed conversion ratio (FCR), there was a significant difference between treatments (p = 0.045); the CONTROL group had the highest FCR, followed by the AGP group, with the best FCR observed in the PROB 3 group (p < 0.05). In addition, significant statistical differences were found between CONTROL and PROB 2 (p < 0.05). Likewise, a significant linear effect of increasing doses of the probiotic was found (p = 0.01), which indicated that the feed conversion was better with a higher dose. Conclusion: The treatments evaluated in this study significantly impacted the FCR in guinea pigs for fattening. Increasing doses of probiotics had a linear effect on FCR.