DEAR EDITOR, Although they have different biochemical structures, macrolides, lincosamides (including clindamycin) and streptogramins (including pristinamycin) share a similar mechanism of action on Gram-positive bacteria, and are grouped into the MLS family. 1 Cross-allergies induced by drugs of similar mechanism of action but different chemical structure (polysensitivity) are poorly described. Our objectives were to investigate the possibility of polysensitivity among MLS antibiotics, and to compare the value of patch tests (PTs) in MLS-induced delayed cutaneous adverse drug reactions (DCADRs).This retrospective, monocentre study was conducted between 1997 and 2016. We included all patients referred for a DCADR (excluding late urticaria) with any MLS drug suspected, alone or with other culprit drugs, with PTs performed after this DCADR. Cases attributed to another drug by a positive PT and those with a negative rechallenge of the culprit MLS were excluded. PTs for erythromycin, clarithromycin, pristinamycin and clindamycin, diluted at 10% in petrolatum, were supplied by Chemotechnique Diagnostics (Vellinge, Sweden). Those for roxithromycin, azithromycin and spiramycin, diluted to 30% in petrolatum, were prepared by our hospital pharmacy, according to guidelines, and were read after 48 and 96 h. 2 Between 1997 and 2012, PTs involved the suspected drug(s) only. After 2012, all patients underwent complete investigation for all MLS drugs. The following data were obtained from charts: age, sex, type of DCADR, other culprit drugs and PT results. Prior or subsequent intake of any of MLS antibiotic was recorded by phone call to reachable patients.Polysensitivity among MLS antibiotics was investigated by two methods: polysensitivity by PTs (positive PT to at least two MLS antibiotics) and clinical polysensitivity (eruption with another MLS before or after the DCADR investigated by PT). Statistical analyses involved v 2 -test or Fisher test as appropriate. Institutional review board approval was obtained (CLEA-2017-42).Seventy-one patients were included (49 female, median age 55 years, range 24-95); 34 could be contacted by phone. The median time between the DCADR and PTs was 5 months (range 1 month to 11 years). The types of DCADRs and PT results are shown in Table 1.Polysensitivity by PTs was first investigated. Among the 25 patients with complete PT investigation, two (8%) had British Journal of Dermatology (2018) 179, pp978-979 supervised oral challenge is warranted to allow for safe further intake of these commonly used antibiotics, which have few effective and cost-effective alternatives.