The eco-toxicological impact of copolyesters during composting was evaluated by plant growth tests with cress (Lepidium sativum) and barley (Hordeum vulgare). The research was conducted to determine if PET beverage bottles modified by lactic acid (copolyesters) and products of their biodegradation would affect compost. The results demonstrate that the composts on which the examined materials were degrading were not toxic to the plants. Compared to the reference sample, the germination and growth of plants were stimulated. The plants showed an increase in plant biomass. Changes in appearance, retarded growth, or necrotic changes were not observed. The resulting compost did not exhibit any unfavorable influence on compost quality, and the products of degradation affected the growth and development of plants positively.