Mild clinical disease was produced in conventionally reared calves by the intranasal inoculation of 18-hour cultures of Pasteurella haemolytica simultaneously with Mycoplasma bovis; at necropsy seven days later moderate pneumonic consolidation was observed in two of four calves. Additional intratracheal injection of these organisms did not increase the severity of disease. In contrast, inoculation of six-hour cultures of P haemolytica with M bovis produced more severe disease and more extensive pneumonic consolidation. The most severe disease and greatest degree of pneumonic consolidation was induced by intranasal and intratracheal inoculation of six-hour cultures of P haemolytica one day after the intranasal inoculation of M bovis. Omitting the intranasal injection of P haemolytica reduced the severity and consolidation only slightly. Studies in gnotobiotic calves revealed that more severe disease and more extensive pneumonic consolidation resulted when M bovis was inoculated before P haemolytic rather than vice versa.