This review examines two putative biological markers for schizophrenia: reduced blood platelet monamine oxidase (MAO) activity and impaired smooth pursuit eye movements. Studies of these biological markers among patient samples are presented, including their theoretical background, measurement, genetics, validity, and specificity of the markers for schizophrenia, and the artifacts that might lessen their utility. These results are then compared with those from the biological high-risk research strategy, which selects nonpatient volunteers solely on the basis of a deviant marker, and then examines the clinical, psychological, and biological correlates of the marker. The results of these studies suggest that low platelet MAO activity is not an adequate marker for schizophrenia but is associated with characteristics related to hypomanic behavior and sensation seeking. Smooth pursuit eye tracking impairment, in contrast, seems to be directly associated with schizophrenia-related traits, such as the negative symptoms found among chronic schizophrenics, or with the characteristics observed in the biological relatives of schizophrenics. Finally, the implications of these findings are discussed.