background: In breastfeeding patients with chronic inflammatory rheumatic diseases, a postpartum flare may require the use of biologics. However, data on the safety of biologics during lactation are scarce, potentially impeding the decision-making process. methods: We report two cases of women in whom treatment with a monoclonal IgG antibody (rituximab or canakinumab) was indicated during the lactation period. In both cases, breastfeeding was continued, and drug levels in the mother's serum, in serial breast milk samples and in the infant's serum were measured. Both rituximab and canakinumab showed minimal drug concentrations in breast milk and no detectable levels in the infants' sera. conclusions: The lack of detectable levels of rituximab and canakinumab in the sera of breastfed infants reflects the poor oral bioavailability of these biologics and helps to promote their use in breastfeeding patients.