ABSTRACT. It is analyzed the firms’ search strategies to acquire external knowledge from universities and public research offices (PROs) to complement their innovation process, using as well knowledge from competitors, suppliers or customers. It is conducted an empirical analysis of 2,023 firms in the Spanish manufacturing sector in a panel from 2005 and 2006, using R&D and non-R&D variables. Results confirm the stated hypothesis establishing that human resources are fundamental drivers to explain absorption of external knowledge; moreover, the industrial classification also constrains the cooperation agreements and the R&D efforts matter, although the lower the technology content of the firms, the less its importance. In fact, in the low-tech sector firms, the R&D expenditures are important but with non-R&D variables the cooperation agreement is also explained. Overall, in a sample made of SMEs and low and med-low tech firms not only the R&D variables are important variables to explain the search of external sources of knowledge