- Formation of the HIV-1 core by the association of capsid proteins is a critical, not fully understood, step in the viral life cycle. Understanding the early stages of the mechanism may improve treatment opportunities. Here, spectroscopic analysis (opacity) is used to follow the kinetics of capsid protein assembly, which shows three stages: a lag phase, followed by a linear increase stage and terminated by a plateau. Adding pre-incubated capsid proteins at the start of the lag phase shortens it and increases the rate of assembly at the linear stage, demonstrating autoacceleration and cooperative assembly. Cryogenic transmission electron microscopy is used to probe structural evolution at these three stages. At the beginning of the lag phase, short tubular assemblies are found alongside micron long tubes. Their elongation continues all throughout the lag phase, at the end of which tubes start to assemble into bundles. Based on these results, we suggest a multi-step self-assembly process including fast nucleation and elongation followed by tubes packing into arrays.