Molecular automata are mixtures of molecules that undergo precisely defined structural changes in response to sequential interactions with inputs(1-4). Previously studied nucleic acid-based automata include game-playing molecular devices (MAYA automata(3,5)) and finite-state automata for the analysis of nucleic acids(6), with the latter inspiring circuits for the analysis of RNA species inside cells(7,8). Here, we describe automata based on strand-displacement(9,10) cascades directed by antibodies that can analyse cells by using their surface markers as inputs. The final output of a molecular automaton that successfully completes its analysis is the presence of a unique molecular tag on the cell surface of a specific subpopulation of lymphocytes within human blood cells.