While much organizational socialization occurs through interpersonal interactions, evidence regarding how these processes unfold over time has not been forthcoming. Results from a 14-wave longitudinal study with a sample of 264 organizational newcomers show that support of newcomers from coworkers and supervisors declines within the first 90 days of employment. Early support and undermining had more significant relationships with work outcomes assessed after 90 days of employment than did increases or decreases in support and undermining over that time period, suggesting early support and undermining may lay a foundation for later work outcomes. Proactive behavior partially mediated the relationship between support and more distal work outcomes including withdrawal behaviors. Supervisor undermining was uniquely associated with higher turnover hazard.