We investigated informal workplace learning (IWL) within an under-researched target group: blue-collar workers. IWL is particularly important for these workers because of learning barriers to participation in formal training. Based on meta-analytical conceptualizations and findings, we developed a conceptual framework of antecedents, processes, and learning outcomes of IWL among blue-collar workers (APO framework), following an input-process-output perspective. The results of our structural equation model analysis with N = 702 blue-collar workers from small and medium-sized businesses provided support for seven of eight hypotheses: Personal antecedents, namely "curiosity," "learning goal orientation," and "self-directed learning orientation" were positively related to IWL; organizational antecedents, namely "social support"--containing "supervisor support," "coworker support," and "error-related learning climate"--and, surprisingly, "time pressure" were positively related to IWL; IWL was positively related to three learning outcomes, namely "job involvement," "newly acquired competency," and "organizational citizenship behavior." The findings establish a basis for future longitudinal studies and theory building in workplace learning research, and they provide managers in organizations with guidance to promote IWL.